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SEEING WITH SOUND WAVES

Echoes of

Bats and Men

BY DONALD R. GRIFFIN

ECHOES OF BATS AND MEN

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LIBRARY

WOODS HOLE, MASS.
W. H. 0. 1.

Donald R. Griffin was bom in 1915 in Southampton,
New York. He was educated at Phillips Academy,
Andover, Massachusetts, and at Harvard University
(B.S., 1938; M.A., 1940; Ph.D., 1942), where he was
variously Junior Fellow and Research Associate until
1946. Grifl&n taught physiology and zoology at Cornell
University until 1953, and since then he has been Pro-
fessor of Zoology at Harvard.

His enthusiasm for science began as a boy when he
lived on Cape Cod. "I always found small mammals
enough like ourselves," Griffin says, "to feel that I could
understand what their Uves would be like, and yet dif-
ferent enough to make it a sort of adventure and ex-
ploration to see what they were doing. CoUege courses
plus reading and conversations with an unusually wise
and stimulating group of friends and advisers led my
interests to include the physiological mechanisms that
operate in the bodies of animals and men."

Since it soon became clear to him that many of the
problems of biology might be solved by direct applica-
tion of the methods and instruments of physics, he be-
gan, first, to band bats, then to study and record the
ultrasonic cries with which they navigate. "By a most
fortunate accident," he says, "I was a student at Harvard
College, where, in 1938, one of the few physicists then
actively studying sounds above the range of human hear-
ing was willing to let my bats register their ultrasonic
sounds on his apparatus. This was G. W. Pierce, and a
casual visit to his laboratory with a cage full of bats be-
gan the line of research that forms the subject of this
book.

"In the same years," he continues, "I was also study-
ing migratory birds, first by homing experiments in
which they were carried some distance from their nests
and released. Many of the sea birds studied in this way

(herring gulls, terns, petrels, and gannets) found their
way home. But homing experiments only tell the time
required and the percentage returning at all. So I de-
cided to learn to fly myself and trace the actual routes
flown. I managed to do this with a number of guUs and
gannets, circling in a Piper Cub for as long as ten hours
at a stretch while the bird did its cross-country flying."
During World War II Griflan applied the biophysical
approach to projects for the development of equipment
for the Armed Forces— headphones and microphones for
communications, cold- weather clothing and electric suits
for fliers, and studies of human vision in the infrared
which were basic to the design of the infrared snooper-
scope viewer.

Griffin's work, which has so advantageously combined
physics and biology, has caused him to feel that his own
introduction to biology and physics could have been
greatly improved upon, that his early education encour-
aged the misconception that "physics was the more diffi-
cult and erudite of the two, and that biology was the
catching, naming, and cataloguing of innumerable varie-
ties of animals and plants." His later experience and re-
search have forcibly demonstrated that working simul-
taneously with both sciences yields original and valuable
results. In fact, these studies have, Griflin says, uncov-
ered "new problems faster than I or anyone else has
been able to solve the old ones. I am now beginning to
suspect that Hving mechanisms operate in ways that are
so intricate and marvelous that if we finally understand
them, we will, in the process, have extended the horizons
of physics."

Dr. and Mrs. Griffin and their four children live in
Belmont, Massachusetts.

61P

ECHOES OF ^r7i

BATS AND MEN

Donald R. Griffin

Published by

Anchor Books

Doubleday & Company, Inc.

Garden City, New York

1959

Available to secondary school

students and teachers through

Wesleyan University Press Incorporated

Columbus 16, Ohio

Back cover photograph by OUie Atkins. Re-
printed by special permission of The Saturday
Evening Post, Curtis Publishing Company, 1955.
Cover design by George Giusti. Typography by
Edward Gorey.

Library of Congress Catalog Card Number 59-12051

Printed in the United States of America

THE SCIENCE STUDY SERIES

The Science Study Series offers to students and to the
general public the writing of distinguished authors on
the most stirring and fundamental topics of physics,
from the smallest known particles to the whole universe.
Some of the books tell of the role of physics in the world
of man, his technology and civilization. Others are bio-
graphical in nature, teUing the fascinating stories of the
great discoverers and their discoveries. All the authors
have been selected both for expertness in the fields they
discuss and for ability to communicate their special
knowledge and their own views in an interesting way.
The primary purpose of these books is to provide a sur-
vey of physics within the grasp of the young student or
the layman. Many of the books, it is hoped, will en-
courage the reader to make his own investigations of
natural phenomena.

These books are published as part of a fresh approach
to the teaching and study of physics. At the Massachu-
setts Institute of Technology during 1956 a group of
physicists, high school teachers, journalists, apparatus
designers, film producers, and other speciaUsts organized
the Physical Science Study Committee, now operating
as a part of Educational Services Incorporated, Water-
town, Massachusetts. They pooled their knowledge and
experience toward the design and creation of aids to the
learning of physics. Initially their effort was supported
by the National Science Foundation, which has con-

THE SCIENCE STUDY SERIES

tinued to aid the program. The Ford Foundation, the
Fund for the Advancement of Education, and the Alfred
P. Sloan Foundation have also given support. The Com-
mittee is creating a textbook, an extensive film series, a
laboratory guide, especially designed apparatus, and a
teacher's source book for a new integrated secondary
school physics program which is undergoing continuous
evaluation with secondary school teachers.

The Series is guided by the Board of Editors, con-
sisting of Paul F. Brandwein, the Conservation Founda-
tion and Harcourt, Brace and Company; John H. Durs-
ton. Educational Services Incorporated; Francis L.
Friedman, Massachusetts Institute of Technology; Sam-
uel A. Goudsmit, Brookhaven National Laboratory;
Bruce F. Kingsbury, Educational Services Incorporated;
Philippe LeCorbeiller, Harvard University; Gerard Piel,
Scientific American; and Herbert S. Zim, Simon and
Schuster, Inc.

PREFACE

Physical principles operate, as far as we know, through-
out a universe which has both astronomical dimensions
and a fine grain, some of it close at hand. New horizons
can be large and distant or they may lie in the very small
and commonplace. The unique properties of water mole-
cules present just as interesting, even awesome, phenom-
ena as does the history of stellar galaxies. And in be-
tween, accessible for convenient study, is a delightful
variety of ingenious mechanisms making up the living
bodies of plants and animals. Man has been said to
"stand between the atoms and the stars," and between
molecules and men are to be found many fascinating
applications of physics, broadly conceived. Outstanding
among these are the ways in which living organisms
utilize wave motion of various kinds. Of particular in-
terest is the interplay between sound waves and the
animals and men who use them.

Sound waves can teU us a great deal about the world
around us, and they are often used for this purpose by
both animals and men. Sound exhibits all the properties
of wave motion, and these properties can be observed
whenever sound travels back and forth from place to
place carrying information about the things it touches.
This is obviously true when people talk to one another
or when birds call from the treetops. But sound waves
are also useful as messengers when only one person or
animal is present to broadcast them and listen for their

PREFACE

echoes a short time later. It is especially stimulating to
examine the many effective ways in which animals make
use of echoes, and to compare these with artificial de-
vices which operate on the same basic principles. This
comparison illustrates the important fact that some of
the most difficult scientific questions have been solved
by co-operation between different branches of science
or technology. A century-old mystery of zoology was
largely dispelled by one afternoon in the appropriate
physics laboratory. And physicists faced with discourag-
ing practical problems are inspired to believe that their
tasks are not quite hopeless when they consider the
accomplishments of even the smallest living brains.

Finally, there is the hope, still far from realization,
that full and proper use of the physics and biology of
echoes may serve to lessen the handicap of blindness.
For what blind men attempt crudely, in fijiding their way
about in a world of darkness, specialized animals ac-
complish widi truly marvelous skill and eflBciency. Elec-
tronic instruments also accomplish the seemingly impos-
sible by detecting invisible targets at great distances.
There is an important unity in the role which echoes play
in the biology, psychology, and physics of orientation.

This account of fruitful co-operation among many dif-
ferent kinds of scientists has drawn upon much of their
published work. Some of this material is discussed at
greater length in Listening in the Dark, and I am grate-
ful to the editors of the Yale University Press for per-
mission to use part of its subject matter. Readers in-
terested in more detailed information will also find
helpful references in the short bibliography on page 147.
I have received many helpful suggestions from the staff
of the Physical Science Study Committee. A large num-
ber of companions and colleagues have participated in
my own observations and experiments, and their aid and

PREFACE

encouragement were essential for the experiments de-
scribed in Chapters 1 and 4. Finally, I am happy to
acknowledge the patience and understanding of my wife
and children who gave up many activities they would
have much preferred to listening to my typewriter.

1 1

CONTENTS

The Science Study Series 7

Preface 9

1. VOICES OF EXPERIENCE 17

Echo Experts in the Ocean— Echo Experts in
the Air

2. ECHOES AS MESSENGERS 35
The Nature of Sound Waves— Echoes We
Seldom Notice— Water Waves and Surface
Echoes

3. AIRBORNE ECHOES OF AUDIBLE
SOUNDS 57

The Acoustics of Clicks and Echoes— The
Velocity of Sound Measured by Means of
Echoes

4. THE LANGUAGE OF ECHOES 83

Orientation Sounds of Bats— Echoes of In-
sect Prey— Precision of Echolocation— Bread
upon the Waters— Resistance to Jamming

5. SONAR AND RADAR 107
Echoes under Water— Prospecting by Echo-
Echoes versus X-rays— Radar— Relative Effi-
ciency of Bats and Radar

CONTENTS

6. SUPPOSE YOU WERE BLIND 129

The Sense of Obstacles— Guiding Echoes

Further Reading 147

Index 151

ECHOES OF BATS AND MEN

CHAPTER 1
Voices of Experience

Doing something in the dark is ahnost always difiBcult;
the darker it is the more troublesome an otherwise sim-
ple task becomes. Worst of all is to be bUnd. It is also
a formidable task to build machines to trace the move-
ments of distant objects which we cannot see— airplanes
flying above the clouds or submarines hundreds of feet
below the surface of the ocean. Finding your way on a
dark night is obviously related to the problems of re-
adjusting to a life of blindness, and instruments for
searching out invisible targets must solve similar prob-
lems. All these solutions are based on the sending out of
some form of energy and the sensing of a part of this
energy as it echoes back from the object at a distance.
When we wish to learn about a difficult subject, such
as the use of waves for searching out the invisible, we
naturally look first for an expert who can explain its
complexities for our benefit. There are experts who have
extensive practical experience in the use of echoes. Some
of them make their hving using echoes to locate small
moving objects which they cannot see. One group are
the physicists and engineers who design and operate

ECHOES OF BATS AND MEN

radar and sonar systems, complicated mechanisms which
send out radio waves or sound waves to locate objects
that return echoes of these probing signals. These sys-
tems will be discussed later, but the present chapter will
be devoted to another group of experts who can draw
upon a longer history of reaUstic, operational experience
—experts who use echoes not only to find their way but
also to obtain their daily bread and butter. If their sys-
tems should fail, they would starve to death, and this
pressure of necessity has led to great refinement and re-
habihty of their methods.

These experts are animals which Uve where sound
replaces light as the best means of finding their way-
caves where bats fly by the thousands, or dark waters
where light is nearly nonexistent or is so difl^sed that
clear images over any distance are impossible. The best
known of these animal experts are the whales and
porpoises, which often swim in dark or turbid waters,
catching fish they cannot see, and the bats, which fly in
near or total darkness, getting all their food by aerial
interception of invisible flying insects. To have survived
at all requhred of these animals and their ancestors
enormous skill at echolocation, the location of objects
by their echoes. By studying the sounds they use and
how they modify them for particular problems of echo-
location, we may learn much that can help blind peo-
ple. Even aside from this reason, we will find these
animals' use of echoes to be a fascinating subject in its
own right.

Echo Experts in the Ocean

Only in the clearest water does fight travel far enough
in straight fines so that objects can be seen at more than
a few feet. Dayfight cannot penetrate nearly to the bot-

VOICES OF EXPERIENCE

torn of the ocean, though this does not mean, as people
used to believe, that the ocean depths are totally dark.
Oceanographers have recently discovered that lumines-
cent animals are so numerous that a suflBciently sensitive
light meter can register the flashes of Ught they give off
when the meter is lowered far below the deepest pene-
tration of sunlight. On the other hand, many rivers and
lakes contain enough sediment so that clear vision is im-
possible for more than a few inches even in daylight.
Yet hosts of fish and other aquatic animals Uve active
lives in these waters where vision is nearly impossible,
and it is not surprising that some have turned to sound
as a medium of communication and a means of orienta-
tion, for sound travels farther in water than does light.

We usually think of the oceans and deep fresh-water
streams and lakes (those without outboard motors) as
silent, and few people even realize that fish or whales
can hear. The chief reason is that our own hearing mech-
anism is designed primarily for use in air and so does
not function well in water. Our ears can detect an air-
borne sound so faint that it approaches the noise level
rising from random motion of molecules. The eardrums
and the chain of little bones and elastic tissue that con-
vey sound waves to our inner ear mechanisms are beau-
tifully adapted to accept sound waves arriving through
the air, but poorly suited to receive them from the water.
When we do hear sounds under water, much of the
acoustic energy flows directly from the water through our
bodies, which are largely composed of water, to the
sensitive portions of the iimer ear, where minute vibra-
tions stimulate the auditory nerve.

Sound waves do not move easily from air to water,
or vice versa. The boundary between a gas and a liquid
acts as an almost impenetrable barrier, and more than
99 per cent of the sound energy is reflected back into

ECHOES OF BATS AND MEN

whichever medium conveyed it to the surface. That is,
airborne sound waves are reflected back ahnost totally
from the water, and underwater sound is equally well
reflected back downward from the surface. Even if we
dive beneath the water, we do not hear as well as fish can.
This helps to explain why the noises made by certain fish
and whales are so seldom noticed, though they have been
known for centuries to fishermen and whalers. Even
biologists have been slow to realize that fish can hear
underwater sounds. Nevertheless, all fish have inner ears
basically similar to our own, and while the sound waves
reach these auditory sense organs by different routes
(through the body itself rather than through air-filled
canals), they stimulate the auditory nerve in very nearly
the same way.

At frequencies up to about 1000 cycles per second
(c.p.s.) the minimum amount of sound energy audible
to a catfish is below the minimum energy detectable by
the human ear. This includes the range of many musical
instruments and the fundamental pitch of the human
voice. At higher frequencies fish are less sensitive to
sounds than we are, but their hearing is not inferior to
that of land animals in any basic way.

An ability to hear underwater sounds is still far re-
moved from the bUnd man's problem of learning how
to use echoes for obtaining more information about his
surroundings. With fish there is only suggestive evidence
that certain species may utilize echoes. But marine
mammals, the whales and porpoises, are not only more
closely related to ourselves but also have almost as highly
developed brains. Their cerebral hemispheres rival ours
in size and complexity. Porpoises, which are no larger
than a man, have extremely well developed inner ears
and equally prominent auditory areas within their brains.
Nor are they silent creatures. Once proper equipment

VOICES OF EXPERIENCE

was available for converting underwater sound to audi-
ble, airborne sound, porpoises were found to be posi-
tively garrulous. An individual porpoise has a large
"vocabulary" of squeals, whistles, grunts, and rasping,
clicking sounds. While fishermen and whalers had heard
some of these sounds from time to time, it was only
during the last war that underwater listening became re-
fined enough and common enough to reveal the immense
variety of sounds used by the marine mammals. Many
of these sounds may be calls for signaling back and forth
from one porpoise to another, but some are clearly used
for echolocation.

In recent experiments individual porpoises have been
isolated in small ponds or experimental tanks, such as
those at the larger marine aquaria of Florida and Cali-
fornia. When obstacles are set up in such a tank, the
porpoises are able to dodge them at high speed, even
when the obstacles are put into place on the darkest
nights. While doing this, porpoises make sounds of vari-
ous sorts, usually faint clicking sounds that were over-
looked at first because they were masked by incidental
noises in the ponds. Most porpoises spend their fives in
the open ocean, but there are a few smaller kinds which
live in the larger and often very muddy rivers, such as
the Amazon in South America and the Ganges in India.
These animals must often thread their way among un-
derwater obstructions, such as logs and fallen trees.
Even the species that five in open waters continue their
activities at night. All porpoises feed on fish, which they
must catch by active pursuit, much of the time in poor
fight where it is impossible to see clearly more than a
few centimeters. Therefore, it is not surprising that the
most impressive feats of underwater echolocation have
been exhibited in the capture of fish by hungry porpoises.

Captive porpoises are usuaUy fed by tossing dead fish

ECHOES OF BATS AND MEN

into their tank— they soon learn to swim directly to the
splash from wherever they may be. The eager approach
of the hungry porpoise could be explained as a simple
localization of the "loud" splash which they had learned
meant food. But two careful experimenters, William
Schevill and Barbara Lawrence (Mrs. Schevill), work-
ing at the Woods Hole Oceanographic Institution,
Woods Hole, Massachusetts, in 1955, noticed that their
captive porpoise found small, silent bits of food by echo-
location. The porpoise spent much time searching the
pond for food, and in doing so he emitted faint creak-
ing noises which could be detected only with sensi-
tive underwater listening equipment. They were not au-
dible to a person listening from the bank of the pond
or to a swimmer with his head under water. The creak-
ing consisted of a series of clicks repeated at varying
rates, sometimes so fast as to become a grating rasp or
buzz. Suspecting that the animal might be listening for
echoes from fish, Schevill and Lawrence sought to learn
whether he could detect and recognize a small dead fish
by echolocation and, if so, at what approximate dis-
tance. To eliminate vision, they frequently worked on
dark nights, and in any case their experimental pond,
only about 20 meters (about 65.6 feet) in diameter, was
stirred into a muddy soup by the constant swimming of
the porpoise. Even a brightly painted piece of metal be-
came invisible in bright sunlight when immersed to a
depth of about 60 centimeters (about 23.6 inches).

When a man sitting in a small boat tied to the shore
quietly held a dead fish a few centimeters under water,
the porpoise learned to swim toward it, "creaking" aU
the time, and seize the morsel. To make the test more
critical as far as the distance of detection was concerned,
a fish net was placed perpendicular to the bank, as
shown in Fig. 1. The net extended out 2.4 meters from

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ECHOES OF BATS AND MEN

the boat, and the porpoise had to decide at more than
that distance which side of the net to choose in swim-
ming up to the boat where food might be expected. He
was fed irregularly, sometimes at one end of the boat,
sometimes at the other, but in cruising past under water
on a dark night he would almost never turn in closer
to the bank than the end of the net unless a fish was
being held beneath the surface. If the porpoise was not
"creaking" as he swam past, he did not swim toward
the boat even when a fish was offered.

For the most significant experiments Schevill and
Lawrence sat at opposite ends of the anchored boat,
each holding a fish at arm's length as the hungry por-
poise swam past through the dark and murky water.
Sometimes one and sometimes the other would gently
and silently sHp a 15-centimeter fish just below the sur-
face, and if the hungry porpoise was "creaking" as he
passed by, he would usually swim in to pick up his fish.
In about three quarters of the tests he would choose the
correct side of the net, even though he had been ac-
customed to pick up his food about equally often at
either end of the anchored skiff.

This and other experiments show that porpoises
can do more than simply detect isolated echoes of
their creaking sounds from objects as small as 15-
centimeter fish. More impressive still, they can discrimi-
nate such echoes from all the other echoes that are re-
turning from the bottom of the pond, the surface of the
water, the bank, the net, the bottom of the skiff, to say
nothing of the rocks and aquatic vegetation.

If a porpoise can echolocate a 15-centimeter fish,
what prevents a blind man from hearing echoes from
objects of similar size that He on the floor or on a table?
Actually there are several factors which work to the
disadvantage of the porpoise. Sound travels about four

VOICES OF EXPERIENCE

and one half times faster in water than in air, so that
the time differences between echoes from objects at dif-
ferent distances are that much smaller and presumably
that much harder to detect. Furthermore, fish are very
similar to water in the way they affect sound waves, and
most of the energy of underwater sound that strikes a
fish continues through its watery body just as though no
fish were there. The same physical problem is present in
the body and hearing apparatus of the porpoise; it too
is nearly "transparent" to underwater sound, and it is
intrinsically difficult for sound waves to interact with
the various parts of its body. Indeed, it is likely that in
the experiments of Schevill and Lawrence the echoes
used by the porpoise came not so much from the 15-
centimeter fish as from its small air-filled swim bladder,
which reflected sound much as an air bubble would do.
It would take us too far afield to present all the physical
properties of underwater sound that are important for
this extremely skillful echolocation by porpoises, but the
interested reader may refer to Chapter 10 of Listening
in the Dark (see Further Reading).

Two facts may help to explain the precision with
which porpoises detect echoes from fish: one is the wide
frequency range of their emitted sounds, and the other
the range of their hearing. Tests have shown that they
hear sounds as low in frequency as 150 c.p.s. and as
high as 150,000 c.p.s. Yet these may not be the true
limits of their hearing but only those set by the apparatus
used to test it. In water where the velocity of sound is
some four and one half times that in air, sound waves
of 150,000 c.p.s. have a shorter wave length than the
highest frequencies to which human ears have really
useful sensitivity. Except by young children, it is doubt-
ful whether frequencies above 15,000 c.p.s. are heard
we'll enough to be useful for detecting objects by their

ECHOES OF BATS AND MEN

echoes. The tenfold increase in the highest audible fre-
quencies shghtly more than offsets the increase in wave
length of sound in water. Thus we should expect the
porpoise to have only about a twofold advantage over
a bhnd man owing to the shorter wave lengths that he
within his repertoire of echo-generating sounds. Factors
other than wave length must therefore explain the ex-
pertness of porpoises at the art of echolocation, which
blind men, as we shall see in Chapter 6, also try to cul-
tivate. Perhaps they have simply learned individually
to pay more attention to echoes, or perhaps in their long
evolutionary history they have acquired ears and brains
that are better adapted in some way we do not under-
stand for sorting out echo components from the complex
mixture of sounds bombarding their ears.

Echo Experts in the Air

Porpoises are large, spectacular, exotic, and it is rel-
atively easy to accept the fact that they are capable of
doing wonders in their watery world. At large outdoor
aquaria they are trained to perform such circus tricks as
leaping out of the water through burning hoops, catch-
ing rubber balls or dead fish tossed to them by their
trainers, and even throwing something back to a particu-
lar person in the audience. No one who has ever watched
these performances, or even motion pictures of them,
can doubt the inteUigence, agility, or skill of porpoises.
But they do Uve in the water rather than in our medium,
the air. Consequently they seem somewhat more remote
from the bUnd man's problems than the other major
group of animals which make extensive use of echoes in
their daily hves. These are the bats— tiny mysterious
creatures and, let's face it, to many people repulsive
ones. Furry little mammals, resembUng mice except for

VOICES OF EXPERIENCE

their wings, tliey prefer darkness, are quite at home in
the blackest caves and generally encountered only as
unwanted invaders of attic or summer cottage. At first
glance nothing would seem to be more remote from any
humanitarian contribution to the biophysics of orienta-
tion of blind human beings.

It is the startling strangeness of bats, plus the folklore
that couples them with demons and the nether regions,
which makes it so hard to think of them with anything
other than repugnance. But they are experts in the use
of echoes, and if we wish to find out what can be learned
about objects from echoes, we must be prepared to ac-
cept important evidence regardless of our feelings about
its source. It would be a real oversight to ignore the skills
attained by bats in guiding their rapid flight by means
of echoes.

Our knowledge of bat navigation really started in
1793 when the brilliant Italian scientist Lazzaro Spal-
lanzani became interested in how various animals found
their way about in darkness. Owls and other nocturnal
creatures, he found, were relying on their large eyes, and
they became helpless in complete darkness. But when
he experimented with bats, he was puzzled to discover
that they continued to fly almost perfectly when they
could not possibly see a thing. Not content with experi-
ments in which they flitted unconcernedly through the
darkest chambers he could find, he finally resorted to
blinding several bats. Even then they flew as well as
ever. He released a number of blinded bats out of doors
and looked for them four days later in the beU tower of
the cathedral at Pavia, where he had caught them for his
experiments in the first place. Wishing to know whether
they had been able to continue their ordinary activities
without their eyes, he climbed up to the bell tower early
in the morning just after the bats' usual time for retum-

ECHOES OF BATS AND MEN

ing from a night of active flight and food gathering. Like
all bats that are found in temperate cUmates, these fed
exclusively on insects, flying insects which they had to
pursue and catch on the wing. Spallanzani caught four
of the bats he had blinded a few days earher, and on
dissecting them found that their stomachs were just as
tightly crammed with the remains of insects as the other
bats, which had not been bUnded.

Spallanzani performed a number of other experi-
ments, which, together with those of the Swiss biologist
Charles Jurine, led him to conclude before his death in
1799 that, while bats could dispense with their eyes, any
serious impairment of their hearing was disastrous.
When their ears were plugged, they coUided blindly and
at randdta with whatever obstacles were set in their way.
Only a really tight closure of the ear canals sufi&ced to
produce total disorientation, but Spallanzani's experi-
ments were completely convincing. One example of the
ingenuity of his methods was the way he investigated the
possibility that the bats' navigation might be disturbed
by irritation or injury caused by the earplugs rather than
by interference with hearing. He had some tiny brass
tubes constructed and fitted them into the ear openings
of the bats. This was no easy job in the 1790s since bats'
ear canals are less than one millimeter in diameter. When
the tubes were in place but open, the bats could still fly
with almost normal skill. When the same tubes were
tightly plugged, they caused no greater irritation, yet the
bats were now wholly disoriented and bumped at ran-
dom into every obstacle. No matter which of several
methods he used to close the ear openings, if the closure
was a tight one, the bat was helpless.

On the other hand, a wide variety of other experi-
ments disclosed no effect of impairment of other sense
organs— vision, touch, smell, or taste. But these findings

VOICES OF EXPERIENCE

seemed to make no sense, for the bats were completely
silent as far as anyone could tell, both before and after
they had been subjected to these various experimental
treatments. How could the ears replace the eyes in guid-
ing their flight? In 1800 there seemed to be no answer
to this question, and Spallanzani's findings were rejected,
ridiculed, and almost totally forgotten. Armchair critics
surmised that some refined sense of touch, probably
located on the wing membranes, accounted for bats'
ability to detect objects at a distance and thus avoid
them, but no one even tried to explain how Spallanzani's
four blinded bats had filled their stomachs with flying
insects.

What came to be called "Spallanzani's bat problem"
was not solved until about twenty years ago after elec-
tronic apparatus had been developed at Harvard by the
physicist G. W. Pierce to detect sounds lying outside the
frequency range of human hearing. Just as soon as I
brought some bats to Pierce's apparatus, it became ob-
vious that they were emitting plenty of sound, but that
it was almost entirely above the frequencies that we
could hear. Further experiments, in collaboration with
Robert Galambos, now of the Walter Reed Army Insti-
tute of Research, showed that covering the mouth of
a bat and thus preventing its emission of these high-
frequency sounds was just as effective as plugging its
ears. Both treatments made bats quite unable to detect
large objects or small, and they bumped against the walls
of the room or anything else in their path. In short,
their whole orientation during flight depended upon
echoes of the high-frequency sounds that they emitted
almost continuously while flying about. Because these
sounds have shorter wave lengths, and consequently
higher frequencies, than those to which our ears respond,
the ability of bats to fly in total darkness had seemed

ECHOES OF BATS AND MEN

a complete mystery. But once this simple fact became
known, all seemed clear, at least in its broad outlines.

As a matter of fact, the sounds with which bats guide
their flight are not totally inaudible. While more than
99.9 per cent of the sound energy emitted by bats that
have been studied most thoroughly is at frequencies
above the human range, there is also a faint audible
component. It is so faint that one is likely to suspect the
sounds come from the fluttering of the wings, and, in
fact, they were overlooked by Spallanzani. Whenever a
bat emits a burst of very high-frequency sound which
can be detected by suitable electronic apparatus, one
can also hear a faint tick accompanying each of them.
Perhaps some readers may have an opportunity to watch
and listen to bats on a warm evening. The bats foimd
in temperate climates often roost in crevices in buildings
and fly out every evening between sunset and complete
darkness. If one stands close to where they fly (1 to 2
meters), and if it is really quiet and you can refrain
from squealing, you can hear these ticking sounds. The
younger you are, the more easily you can hear them,
for even the audible component has a frequency of
roughly 5000 to 10,000 cycles per second. There are
also a few kinds of tropical, fruit-eating bats which make
clearly audible ticks when they fly in dark caves. Where
there is any light they use their eyes, which are much
larger than those of the insectivorous bats. Two kinds of
cave-dwelling birds also click loudly when flying in com-
plete darkness but rely on vision at other times.

The faint ticking component of a bat's orientation
sound is of very short duration, not imlike the ticking
of a lady's wrist watch. But, unlike a watch, the bat's
ticks will vary markedly in their tempo. If it is flying
straight at some obstacles from a distance, there may be
from five to twenty ticks per second. But if the bat is

VOICES OF EXPERIENCE

faced with complicated navigational problems, such as
dodging you or a stick which you hold up in front of
you, you may hear the ticking suddenly increase imtil it
forms a faint buzz. The same thing happens just before
a bat makes a landing, but the audible ticks are such a
faint sound that it requires patience and completely
quiet surroundings for them to be heard. The auditory
basis of obstacle detection by bats was independently
recognized in 1932 by a Dutch zoologist, Sven Dijkgraaf,
who made a careful study of these faint, audible clicks
and noted how closely they were correlated with the
echolocation of obstacles. This is an example of the need
for care, patience, and appropriate conditions if one is to
notice and enjoy some of the more fascinating facets of
the natural world.

Bats are not always agile and clever fliers; sometimes
they are sleepy and clumsy— especially when they have
been disturbed in the daytime. Most American and Eu-
ropean species tend to let their body temperature fall to
about that of the air in which they sleep. In winter many
kinds of bats hibernate in caves or other places where
they find temperatures only a few degrees above freez-
ing. At such temperatures they are completely torpid,
and one may easily think them dead. In between deep
hibernation and full activity are many degrees of activity
and alertness. The bats we are most likely to find and
have an opportunity to observe are usually those that
are least agile and least ready to display their full reper-
toire of flight maneuvers. If fully airworthy, they would
be xmHkely to let us watch them for long at close range.
Furthermore, they often seem to become tired, and when
chased about a room or attic they may soon become
clmnsy from fatigue alone. But if one takes the trouble
to observe bats at their best, when fully awake and in
top flying condition, as they are every night of normal

ECHOES OF BATS AND MEN

summer insect hunting, then their agility and finesse at
flying through comphcated pathways are truly amazing.
Flitting between the rungs of a chair is easy for an ani-
mal which naturally flies between the smaller branches
of pine trees on the darkest nights.

After considering these two expert practitioners of
echolocation and before moving ahead, let us put our-
selves back into the frame of mind in which Spallanzani
must have viewed these phenomena. From curiosity
about the vision of nocturnal animals he had been led
to perform a long series of careful and critical experi-
ments with bats. While he could hear no sound as they
flew about, he had convinced himself, despite his strong
initial skepticism, that the ears and not the eyes were the
sense organs that informed bats about such small objects
as threads strung across the rooms in which he made
them fly. He could make no more sense out of this con-
clusion than could his critics. But he trusted experimen-
tally demonstrated facts sufficiently to be convinced of
the correctness of his findings, even though he could not
fit them into a satisfactory logical framework. This sit-
uation may arise from time to time in any branch of
science, and often it means that some important new
principle is just beyond our grasp. When facts fail to
fit into our theories, there is usually a need to modify the
theories.

Is there any reason to suppose that scientific history
has just recently come to an end? Almost certainly not,
and this inevitably means that new and totally unex-
pected discoveries are going to be made in the future.
The example of Spallanzani and the acoustic orientation
of bats can remind us of several important points. First,
the most rewarding discoveries may be awaiting us in
what seem at first sight the most unlikely places. Second,
accepted theories explaining a phenomenon have often

VOICES OF EXPERIENCE

proved in the past to be mistaken. There is always room
for constructive questioning of even the most well estab-
lished theories. Who knows what current beliefs may
be shown to be as much in need of revision as the
nineteenth-century view that bats felt their way through
dark caves by some sense of touch residing in their
wings?

CHAPTER 2
Echoes as Messengers

Since bats and porpoises leam so much by listening to
echoes, it is important to examine the properties of
sound waves that make them such useful messengers.
Before sounds or other types of wave motion can tell us
anything at all, they must interact with something— the
surface of the earth, the walls of a house, a human lar-
ynx, or the intricate mechanism of an ear that listens.
Only by its doing something to some piece of matter,
directly or indirectly, can wave motion be detected in
the first place. Try to imagine a kind of radiation which
penetrates whatever may stand in its way, traveling on
and on without being changed, distorted, or deviated in
its direction of travel. How could we leam that such
rays even existed? High-energy cosmic rays and the sub-
atomic particles called neutrinos have only the very
slightest effect on matter under ordinary conditions, and,
therefore, they were most difficult to discover and still
are almost impossible to measure with any precision.
Radio waves from natural sources have always existed
at low levels of intensity and they penetrated the bodies
of our ancestors just as they do our own. But only in

ECHOES OF BATS AND MEN

very recent times have men observed the interactions of
radio waves with appropriate detecting instrmnents and
thus learned of their existence. Suppose that everything
in the world were suddenly made perfectly transparent
and, furthermore, that nothing gave off light or caused
it to change direction in passing from one material to
another. In such a world one might as well be blind.
Even though you possessed the only sense organ or de-
tecting instrument capable of receiving a type of radia-
tion that penetrated everything else in the universe, such
special powers would be of Uttle use. Because light and
sound do interact with matter around us, they are types
of radiation for which sensitive detectors are useful. This
is why animals and men have come to possess such
effective eyes and ears.

The Nature of Sound Waves

Wave motion can conveniently be thought about in
pure and continuous form: a sound having a single fre-
quency, for instance 2000 sound waves or cycles per
second, or light of a pure spectral color, for instance the
D line of the sodium arc having a frequency of 5.1 X
10^3 c.p.s. (51,000,000,000,000 c.p.s.). Such continu-
ous waves may be described quite accurately for most
purposes by a graph in which the size or amplitude of
the wave motion is plotted on one axis and time on the
other axis. For sound of a single frequency or a single
spectral color such graphs are smoothly undulating Unes
called sine waves. A sine wave is the graph you would
draw if you plotted the vertical motion of the hand of
a clock as a function of time. Suppose you tied a string
to the end of the hour hand of a wall clock, such as the
one in almost any schoolroom, and tied a light weight
to the other end of the string (Fig. 2). At nine o'clock

ECHOES OF BATS AND MEN

and again at three the weight will be halfway between
its highest and its lowest positions, and you might draw
a horizontal reference line along the wall below the
clock at this level. Now suppose that at other times in
the day you measured the height of the weight above
and below the reference line, calling this distance a, or
the ampUtude of motion of the weight. At noon the
amplitude would be + a and at six o'clock it would be
- a. Every six hours a would be zero, and around noon
and six the curve would slow its rate of rise or fall and
reverse direction. If you used the minute hand instead
of the hour hand, you would plot twelve excursions of
the weight during one revolution of the hour hand.

This same graph would also represent the sine func-
tion studied in geometry and trigonometry. But this
convenient picture of a wave leaves out any possible
interactions with material objects. Hence, we shall find
it necessary to think about waves in somewhat differ-
ent ways or at least add to the simplified concept of a
continuous sine wave before we can use it to deal ef-
fectively with the message-carrying function of sounds.
These special modifications add new interest to the sub-
ject of wave motion as it is considered in physics courses.

What are sound waves and how do they differ from
other kinds of wave motion? When sound is traveUng
through some medium such as air, the pressure of the
medium is changing rhythmically, increasing and de-
creasing at any particular point at a rate which we call
the frequency of sound (Fig. 3). To be sure, these
changes may not be regular, but even when they are too
irregular to be called a single frequency it is still true
that alternating pressure changes occur and that the at-
mospheric pressure fluctuates above and below its aver-
age value, designated as one atmosphere, which one
would measure with a barometer (at sea level the at-

ECHOES AS MESSENGERS

mosphere exerts the same pressure as does a 76-centi-
meter column of mercury). Furthermore, these re-
gions of slightly higher or lower pressure travel through
the air, so that the zone of higher pressure which at
one moment is passing a particular point will later
be found some distance away. These pressure changes
are much smaller than most people realize; for ex-

Fig. 3. If you could photograph the molecules of air
around a source of sound, you would find some mole-
cules grouped closely and others loosely. There is a
high pressure in the close-packed areas and a lower
pressure elsewhere. A graph of this variation in pres-
sure is a sine curve.

ECHOES OF BATS AND MEN

ample, a loud shout varies the air pressure by about
0.00001 to 0.0001 atmosphere, and the faintest sound
that can be heard by a normal human listener has pres-
sure variations of about 2 X lO'^^ (2/10,000,000,000)
atmosphere.

People used to question the existence of sound in the
absence of a human listener. It was debated whether
there was any sound from a waterfall in the wilderness
when no one was there to hear it. This sort of question
ceases to be of much importance once one distinguishes
between the physical phenomenon of sound waves, pres-
sure changes which travel through the air, and the
subjective sensation of hearing a sound. The latter, of
course, requires a listener, although an animal would
do as well as a human being. But unless one believes
that the waterfall and the air around it have wholly dif-
ferent properties when no man is present, it is beyond
question that the physical sound waves are, in fact, gen-
erated as long as the water is falling.

Sound waves travel in liquids and solids as weU as in
gases such as air, and while most of the time we will be
dealing with sound in air, we should bear in mind that
sound waves (that is, moving pressure changes) also
travel through the depths of the ocean or the hardest
steel. There is, however, one great limitation to the
travel of sound waves. They must have something to
travel through and they are barred forever from empty
space or from a perfect vacuum. Pressure results from
the coUisions of molecules with one another and with
whatever surfaces form the boundaries of a gas, liquid,
or solid. Sound can travel at appreciable intensities only
where appreciable pressures exist, and this means where
molecules are close enough together to collide with each
other reasonably often.

The next important fact about sound waves is their

ECHOES AS MESSENGERS

velocity of motion. Once started, they move through a
given medium at a constant rate under any particular set
of conditions. They usually become weaker and weaker
as they progress, and eventually die out. But as long as
they are detectable at all their velocity remains the same.
Nor does the velocity vary with the frequency of sound.
This means that when a sound contains more than one
frequency (that is, the waves have more complicated
shapes than simple sine curves), the different parts of
the complex sound wave move together without one
component lagging or gaining on the others. The actual
speed of sound depends primarily upon the medium
where the sound waves travel, but temperature and other
factors affect it slightly. For example, in air at 20° C
sound travels 344 meters per second (about 1130
feet/ sec), and in sea water at 0° C its velocity is
about 1550 meters/sec (or 4700 feet/sec). While these
distances are fairly large, they are, of course, far less
than the 300 X 10^ meters (or 186,000 miles) cov-
ered in one second by light and radio waves. Hun-
dreds or thousands of meters are less convenient to
think about than shorter distances more comparable to
our own dimensions. Consequently it will often be con-
venient to specify the velocity of sound in terms of dis-
tance traveled in 1 millisecond, or thousandth of a
second; 344 meters/sec is 34.4 centimeters, or about
one foot, per millisecond, a helpful figure to keep in
mind when dealing with sounds of very short duration.
Another important property of a sound is its wave
length or wave lengths. Wave length is the distance be-
tween successive zones of maximum or minimum pres-
sures as the wave travels along. Since the velocity of
sound is constant, the waves, which cover 344 meters
in 1 second, may either be numerous and short or few
in number and longer in wave length. If the waves are

ECHOES OF BATS AND MEN

short, there are more of them in a given distance and
more reach a given point in any particular interval of
time, which is another way of saying they have a higher
frequency. Expressed as a simple equation, velocity
equals frequency times wave length (v = fxX). Or
since the velocity is always the same under a given set of
conditions, the wave length varies inversely as the fre-
quency. A sound wave having a frequency of 344 waves
or cycles per second has a wave length of approximately
1 meter; 1376 c.p.s. has a wave length of 0.25 meter,
and a wave length of 2 centimeters (0.02 meter) cor-
responds to a frequency of 344 ^ 0.02, or 17,200 cycles
per second. High frequencies are often expressed in
kilocycles (thousands of cycles) per second, abbreviated
kc. A sound lasting 1 second, whatever its frequency
may be, extends 344 meters from start to end as it
travels through the air. A cUck lasting only 1/1 00th
second is 3.4 meters from front to back. And a sentence
which takes 10 seconds to utter would extend 3440
meters (more than two miles) from the speaker's mouth
if his voice were strong enough to carry that far. Assum-
ing that the atmosphere is dense enough to carry sound
waves up to an altitude of only 30,000 meters and your
voice loud enough, it is amusing to estimate how long a
sound would have to last in order to make a continuous
series of sound waves from your mouth up to 30,000
meters. It would be 30,000/344 or about 87 seconds,
or 1.5 minutes, roughly the time it would take you to
read aloud half a page from a book.

The interactions of sound waves with ourselves and
the objects around us are less obvious than those involv-
ing light. For example, almost every solid object casts
some sort of shadow if exposed to light shining from
one direction. But most famihar sounds can be heard
with little change if the same shadow-casting object is

ECHOES AS MESSENGERS

Fig. 4. Sound does not cast sharp shadows, but it does
go around corners as well as being reflected back from
solid objects.

placed between the sound source and the listener's ear.
Sound goes around comers more easily, and conse-
quently it is more difi&cult to exclude it from a house or
room or from a piece of scientific apparatus (Fig. 4).
Even though sounds can be reduced in loudness by walls
and other barriers, we seldom think about the degree to
which they are blocked, transmitted, or reflected. So un-

ECHOES OF BATS AND MEN

familiar is this topic that we have no common words
analogous to transparent and opaque to express the fact
that sound waves penetrate a substance easily or not at
all. Nor have we any acoustical equivalents for shiny or
matte to describe surfaces which reflect sound waves
chiefly in one direction or about equally in all directions.

Reflected sound waves are called echoes or reverbera-
tions and they have an important effect on what we hear.
We have seen that certain animals, such as bats and
porpoises, fimd their way by listening for echoes. Blind
men also make use of sound for orientation, and their
dependence on reflected sound waves will be taken up
more fully at the end of the book. But before we go on,
it will be helpful to specify the meaning of a few words
that are useful in describing the message-carrying ability
of sound waves.

Echo generally suggests a distinct, separate reflection
of a sound from some surface at a considerable distance.
Reverberation implies the multiple reflections of a soimd
from surfaces at closer range, so that reflected sound
waves tend to overlap and become mixed with the origi-
nal ones. In a more general sense, however, an echo is
any sound wave that has had its direction materially
changed after striking an object. When reflected waves
travel through the same space as later waves from the
same sound source, they interact and either increase or
decrease the previous level of air pressure. If the pres-
sure at a given point at a given time is increased by the
presence of the echo waves, we say constructive inter-
ference or reinforcement has occurred; if the soimd
pressure is reduced from what it would have been with-
out the reflected waves, we speak of destructive inter-
ference or cancellation. These terms have just the same
meaning for sound waves as for Ught.

It is important to appreciate the relationship between

ECHOES AS MESSENGERS

the velocity of sound, on the one hand, and the distinc-
tion between reverberations and echoes on the other. In
air, where we do most of our Ustening, a sound lasting
one second extends 344 meters through the air, and only
if it is reflected from an object more than half that dis-
tance away (172 meters) will a listener close to the
source receive an echo which begins after the original
sound has ended. Several syllables can be uttered in 1
second— "one thousand one," for instance— and with a
little effort a short syllable such as de can be repeated as
rapidly as five times per second. If one spoke a single
short syllable lasting 0.2 second, an echo would be sep-
arated from the outgoing sound even though the reflect-
ing surface was only slightly more than 34 meters (172
X 0.2) away. It is not often that we hear echoes clearly
separated in time from the original sounds that created
them. This is partly because we seldom deal with single
sounds as short as 0.2 second, or reflecting surfaces as
distant as 34 meters, and also because our ears do not
distinguish two sounds as separate unless there is a frac-
tion of a second of quiet between them. Even when two
sounds are so close together in time that they seem to be
single, the combination usually sounds different from
either of its two parts if they are heard alone. Two clicks
that follow each other too closely to be heard as a double
click sound duller than either one all by itself. Or, if
closer still, the pair of chcks may simply soimd louder
than one alone.

Echoes We Seldom Notice

The echoes that usually follow every word we speak
add to its quality and impact even though we are not
aware of the reverberations as separate and distinct. This
can be illustrated by simple experiments in which some

ECHOES OF BATS AND MEN

constant sound source is carried in and out of doors. A
talkative companion might be one's first choice for a test
source of sound, but he will almost certainly change the
loudness of the conversation on moving from a closed
room to the open air. A portable radio is better, provided
that the building does not contain enough metal to act
as a shield for radio waves. First one might choose an
ordinary frame house for the experiment and set the
volume control of the radio to a level which produces
comfortably loud speech or music as the set rests on the
groimd. Carrying the same radio into a small room
makes it soimd much louder. Not only do the sound
waves reflected from the walls add to the total acoustic
energy reaching our ears, but also the announcer's voice
will seem to change its quaUty because the room has
selective effects on different frequencies of sound.

Of course this experiment is a crude one, complicated
by many pitfalls. Perhaps there were distracting noises
on the street, or at one time you may have stood closer
to the loudspeaker. Perhaps the announcer happened to
talk louder during the time you had the set indoors. A
better experiment might involve some more constant
source— a whistle, typewriter, alarm clock, or other
noise-making machine, a baby's rattle or the louder kind
made for use on New Year's Eve. Best of all, in many
ways, is to use a tape recorder which can be carried
back and forth, indoors or outdoors. In this way you
can use the same sample of speech or music or perhaps
make up a tape recording in which the same sequence
is repeated often enough so that you can listen to it
repeatedly indoors and out. If you are still skeptical,
and you should be, you may wonder whether anything
changes in you, the Ustener, as you move back and forth.
Does your hearing become less sensitive when you are in
the open air? Many careful experiments have shown that

ECHOES AS MESSENGERS

this is not so in ordinary circumstances. Furthermore,
we can make objective measurements of sound intensity
in the two places with a microphone attached to a sensi-
tive voltmeter. Such measurements confirm our impres-
sion that the same source of a continuous sound such
as speech or music does produce a higher sound level
indoors.

Let us pursue the matter a Uttle further and assume
that a tape recorder is available for experiments of this
type— perhaps you can borrow one from a friend or your
school. It will be more useful if you have a long exten-
sion cord, perhaps 50 feet in length, so that the instru-
ment can be operated well away from the building as
well as indoors. What sorts of sounds shall we compare
in the two situations in order to learn as much as we can
about the effects of echoes on how sounds sound with
and without echoes and reverberations? Speech and mu-
sic are excellent to get a better general understanding of
these effects. But no two passages will have the same
assemblage of sound waves, and it will be difficult to
compare the quaUty of the different notes, words, and
syllables in the same recording when heard indoors and
out. With the microphone of your tape recorder you
can record a sustained vocal note or one from any mu-
sical instrument. It is difficult to make a recording which
is really continuous and does not fluctuate in loudness.
But the best solution is to splice the tape into a con-
tinuous closed loop long enough to pass around both
reels so that the machine plays the same sample over
and over again.

This experiment wiU immediately demonstrate one
important effect of echoes in a room. If a shrill,
high-frequency note is maintained at a constant level
in an ordinary room and if one Ustens to it carefully
while moving slowly across the room, its loudness will

ECHOES OF BATS AND MEN

rise and fall at regular intervals of distance. It is of par-
ticular interest to observe this effect while moving slowly
near the middle of a room with the tape recorder at one
end of the room playing a note that is two or three
octaves above middle C. The comparison may be easier
if one ear is covered so that the tone is heard entirely
through the other. Listening in this way, one can usually
hear clearly the waxing and waning of the soimd level
and you can, with care, estimate the distance from one
loud spot to the next. A meter stick hanging horizontally
at about eye level may make it easier to judge the dis-
tance through which the ear must move in order to pass
from one point of maximum loudness to the next.

Having observed these fluctuations in the level of our
recorded tone within a closed room, you repeat the same
experiment out of doors. Not only will the tone sound
fainter but the fluctuations will largely disappear; the
loudness will fall off gradually as one walks away from
the loudspeaker. Such a simple experiment as this will
demonstrate that reflected sound waves from the walls
of the room are interacting with those coming straight
from the tape recorder and that at some places there
is constructive interference producing maximum sound
levels, while elsewhere there is destructive interference
causing zones of relative quiet. Furthermore, it will be
foimd that the distance between the maxima is one half
wave length, provided that tones of nearly a single fre-
quency are used. C2, the second C above middle C, has
a fundamental frequency of 1024 c.p.s., or a wave length
of close to 30 centimeters, and it is therefore a con-
venient frequency to use for such experiments. At a
lower frequency, such as 100 c.p.s., the wave length will

equal or exceed the dimensions of the room (X. = j^ =

344

-T— r = 3.4 meters), and at 10 kc or higher the successive

ECHOES AS MESSENGERS
344

maxima and minima (X = ^^ ^^^ = 0.034 meters or 3.4

centimeters) will be too close together for easy de-
tection. Notes from musical instrmnents have so many
frequencies, or harmonics, each giving its own maxima
and minima at its own wave length, that it will be diffi-
cult to distinguish the loud and quiet spots for each
frequency. Hence, the purer the note the more obvious
the effect. You will find the flute more satisfactory be-
cause of its purer tone than a piano or violin.

These maxima and minima are called standing waves.
A loud spot is the point where sound waves reflected
from the walls add to others arriving directly from the
loudspeaker. If several parts of the walls all send strong
reflections to the same spot, these various echoes are
likely to arrive at different times and fail to reinforce
each other as strongly as they would if arriving at the
same time. In some rooms of irregular shape the standing
waves may thus be inconspicuous, but most rooms are
regular enough and have sufficiently reflective walls so
that at least in the middle of the room the standing-
wave pattern is noticeable. If you have an opportunity
to experiment with a ripple tank in which surface waves
on water are generated to illustrate the various phenom-
ena of wave motion, you will fimd that the frequency of
the vibrating object producing the waves has to be ad-
justed rather carefully to obtain pronounced standing
waves. Otherwise the water's surface may show only a
shifting and confusing mess of wavelets chasing each
other back and forth without apparent order. If the tank
is not a simple shape, such as a rectangle, then the
standing-wave patterns are either very complicated or
are limited to a few areas where reflected waves do man-
age to reinforce those arriving directly from their source.

Suppose we try to set up standing waves in a room by

ECHOES OF BATS AND MEN

generating not a single frequency but a sound containing
many different frequencies. Speech and music answer
this description, but the different frequencies change
rapidly with time, so the effects are compUcated. Still, it
is true that even though we do not ordinarily notice
standing waves of speech or music, in some very large
rooms there may be "dead spots" where interference
between the direct and reflected sound makes hstening
very difficult and unpleasant. Indeed, there is a whole
science of architectural acoustics devoted to minimizing
such "dead spots" and to controlling the echoes from
the walls of auditoria so that speech and music are car-
ried as faithfully as possible to all parts of the hall.

A simple experiment with our tape recorder in an
ordinary room can demonstrate the effects of having
many frequencies present at the same time. A loud hiss
made vocally into the microphone will, when played
back, fill the room with a still louder hiss. But you will
probably have great difficulty in hearing standing waves.
The same experiment can sometimes be performed by
turning up the volume control of a radio or record
player until you hear a hissing sound that incidentally
stems from the random motions of molecules in some
part of the electronic circuit. It has a wide range or band
of frequencies, as does a vocal hiss, and all are about
equally loud. So many different wave lengths are present
that even though each one tends to set up standing waves
at its own wave length, all the others have equally strong
tendencies to estabHsh loud spots separated by their
wave lengths. The result is that the over-all level of the
sound is much the same from point to point within the
room. To obtain clear standing waves there must be
only one or a very few wave lengths prominent in the
sound that fills the room.

Perhaps when you are listening for standing waves

ECHOES AS MESSENGERS

someone else may walk into the room. Often this will
cause a shift in the positions of the maxima and minima
even though the room is fairly large. This is a more
compHcated type of interaction in which the exact loca-
tion of greatest interference is influenced by all sorts of
objects that add reflected sound waves to those arriving
directly from the tape recorder. Because of these changes
the standing-wave patterns could be used to tell us that
someone had entered the room. Usually we have much
easier ways of knowing this, but there are circumstances
where changes in standing-wave patterns have been put
to use to detect small changes in the position of objects
in a room. One type of burglar alarm operates on this
principle. Suppose you were walking about blindfolded
in the same room. The standing- wave pattern would at
least inform you that you were in a room and not out
of doors where nothing was reflecting enough sound to
set up standing waves. It could also tell you when some-
thing else moved into the room, provided you were
standing still and noticed the shift in the standing waves.
These may seem to be trivial examples, but blind men
do learn to pay attention to many aspects of the sound
fields in which they live and in this way learn much
about what goes on around them. Remember, too, that
these examples have been selected for their simplicity,
and from such crude beginnings we can go on to much
more difficult questions that can be answered by carrying
these experiments further. This is in essence to use
sound waves as tools or "sense extenders" for exploring
one's surroundings. Crude tools used with little skill
yield only crude information. But, as we already have
seen, even such small animals as bats have become ex-
pert at using sound waves as tools of this sort to learn
rather complicated facts about what goes on around
them. They have come to do this in the long course of

ECHOES OF BATS AND MEN

their evolutionary history because they live and move
under conditions where sound is a convenient or per-
haps the only available means for maintaining their
orientation.

Telling whether you are indoors or out on the street
by listening to a tape recording of a shrill and monoto-
nous tone may seem a clumsy way to accompUsh the
obvious. But transpose the situation to a man lost in a
pitch-black cave and unable to use a hght of any kind.
Sound waves would be one of the most useful means, if
not the only means, at his disposal to learn about those
parts of the cave beyond the direct reach of his out-
stretched hands and feet. Bats do not feel their way; they
fly rapidly through complex and tortuous passages of a
cave, dodging stalactites and other bats without acci-
dents of any kind, and, as I shall explain later, this is
one of the less difficult of the many tasks these little
animals accomplish by means of sound waves.

In pursuing these matters further it will be best to
return from time to time to the simple experiments with
audible sounds such as those we have just conducted. In
this way you may have firsthand experience to confirm
and support the concepts and theories about which you
read. For many purposes the ripple tank used in physics
courses provides more convenient types of waves with
which the same phenomena can equally well be visual-
ized. This is basically because surface waves on water
travel slowly enough for you to watch them directly.
Furthermore, their velocity varies with the depth of the
water, and they can be caused to bend by installing
shallow "sandbars" or "reefs" in the ripple tank. The
same ripple tank can also be used to study echoes which
are closely analogous to those that cause standing waves
of sound and to those used by bats or men to find their
way about in situations where hght is not available.

ECHOES AS MESSENGERS

Water Waves and Surface Echoes

There are many and detailed parallels between water
waves and sound waves in air or, for that matter, light
waves; but aside from their serving as slow-motion
models, we are likely to think of surface waves on water
as of little interest and certainly as wholly devoid of the
abiUty to carry information. Who would think of trying
to signal back and forth across the ocean by means of
water waves? They die out too soon and are too easily
confused with the natural waves from winds or water
currents. A leaf that falls to the surface of a quiet pond
may produce a few ripples, but how could one hope to
detect this event a hundred feet away? Yet from their
very similarity to sound or light we might expect the
water waves we study in the physics laboratory to have
some message-carrying function. Such cases can be
found if we look for them in nature and, in this instance,
the search leads to the so-called whirligig beetle which
bridges the gap between the ripple tank and the most
compUcated radar installations.

Whirligig beetles are common inhabitants of small
ponds and quiet streams. While these aquatic insects
often dive and swim below the surface, they are usually
noticed most easily when darting about on the surface
film of the water. They are light enough in weight so
that they are supported by the surface tension of the
water— largely because of their fringe of hairs covered
with a thin film of waxy material that does not readily
become wetted. This ability to support themselves on
water could easily lead us into a digression about sur-
face tension and why water is a uniquely suitable liquid
for the flotation of water beetles. But this is a subject

ECHOES OF BATS AND MEN

which is covered well in the Science Study Series book
Soap Bubbles,

More pertinent is the fact that the water beetles make
use of surface waves to keep themselves posted about
the proximity of the water's edge. They have eyes and
use them under many conditions, but at night or when
vision is prevented by laboratory experiments performed
in darkness they still manage quite skillfully to avoid
collisions with the edge of an aquarium and with each
other. A German biologist named Friedrich Eggers

Fig. 5. A whirligig beetle whose other legs lie beneath
its body is able to sense water waves and their echoes
with the two specialized antennae which protrude from
the head and float on the water. It is also interesting
to note that this beetle has four compound eyes, two
above the water and two below.

Studied these beetles with great care in the 1920s. Un-
like those of most insects, their antennae, or feelers are
shaped in an especially suitable way to float on the sur-
face film of the water. The numerous hairs all arranged

ECHOES AS MESSENGERS

parallel to each other are at just the correct angle so that
they float in the surface film. But more specialized still
are the hairs located on one particular joint of each an-
tenna, the second from the base. These specialized hairs
are more than flotation devices; at the base are sensitive
nerves that are stimulated by the most minute move-
ments of the hairs relative to the remainder of the beetle.
Eggers surmised from the microscopic structure of these
hairs and nerves (see Fig. 5) that they were used to
detect motion of the water surface, and he therefore
experimented with them directly. In some beetles he
damaged the second segments of the antennae, cut
off the hairs on this portion, or damaged only the
nerves leading from the bases of these hairs into the
central nervous system of the insect. When these water
beetles were placed on the surface of an aquarium in
the dark, they acted as bewildered as a bird fluttering
against a windowpane and collided at random with the
walls.

Other experiments have shown that the sense organs
of insects can respond to very weak vibrations. A move-
ment of as little as 4 X 10"® centimeter is detected by
the sensory nerves attached to fine hairs on the surface
of some insects which are generally similar in structure
to the whirhgig water beetles. There is thus no reason
to be amazed that water beetles can feel the surface
waves generated by their own swimming or walking
movements. What is amazing is their ability to discrimi-
nate the jiggling that results from reflected waves from
all the other vibrations that must be affecting the same
hairs and the same sensory nerves. This is a problem
which the beetles may avoid to a considerable extent by
their habit of swimming intermittently, with frequent
pauses during which they may perhaps be feeling the
"reverberations" of the water waves their swimming has

ECHOES OF BATS AND MEN

generated a fraction of a second earlier. But the naviga-
tion of water beetles has not been studied since Eggers'
day, and it is typical of the opportunities that await
patient and ingenious students of biophysics. In the fol-
lowing chapters I shall describe in more detail better
known examples of animals' and men's learning a great
deal by listening for echoes, and it will become apparent
that living sense organs and brains detect echoes that
seem on first thought far too faint to be of any possible
use. The phenomenon is basically one of discrimination,
or sifting out faint but important echoes from much
stronger waves of the same type which are not relevant
for the purposes of the particular animal. Living nervous
systems are superior to artificial machines in making a
wide variety of fine discriminations, and the next chap-
ter describes experiments you can perform yourself to
show how the human ear and brain discriminate among
various types of sounds including echoes.

CHAPTER 3
Airborne Echoes of Audible Sounds

The word "echo" suggests a quiet country scene where
a steep cliff or hillside looms up hundreds of feet away.
A shout or a gunshot suddenly breaks the silence, and
there follows a repetition of the sound, fainter than the
original. Knowing the velocity of sound, we could de-
termine our distance from the hillside if we accurately
measured the interval of time from the onset of the out-
going sound to the arrival of the first echo. This can be
done with a stop watch, provided that the hill is large
enough and distant enough so that a clearly audible echo
will return after some seconds. K the hill is too close,
the time interval will be too short for easy and accurate
measurement; if it is too far away, the echo may not be
audible at all. Often there are too many hills producing
multiple echoes, and if the first of these overlaps the
end of the outgoing sound or there are reverberations
from objects in our immediate vicinity, then the accurate
measurement becomes difficult. Nor is it always easy to
decide just which hill is sending back the echo; in fact,
the easiest procedure often is to time the echo and then
scrutinize a large-scale map in search of a steep hillside

ECHOES OF BATS AND MEN

at the correct distance. And there are ahnost always
other sounds to compete for our attention. Thus obvious
echoes have come to seem rather special sounds to be
heard only in the most favorable circumstances.

One situation where echoes have been put to practical
use is aboard boats in foggy coastal waters. Usually it
is quiet in a fog and, aside from the boat itself, no
echoing surface interrupts until the shore is reached.
Often fishermen who find themselves in foggy waters
and think that steep shore lines or cliffs may be within a
mile or so produce a clear echo by making a short, loud
sound. Sometimes this is a blast of a horn or whistle,
required by law in any case to signal their presence to
other boats, or the probing signal may be simply a shout.
Some fishermen say they can even hear echoes from
channel-marker buoys (about three feet in diameter)
at several hundred feet. The usefulness of this method
of navigation is often limited by the lack of adequate
echoing targets in the air above the actual underwater
hazards. Rocks need not reach the surface to be dan-
gerous, and most shore lines are too gentle to provide
reliable echoes.

Modem instruments have largely supplanted air-
borne sound by transposing the same basic process into
the water itself. Sound waves are broadcast from the
boat's hull, and echoes of underwater sound from the
bottom or from shoals ahead of the boat are recorded
by instruments. Such devices for echolocation under
water are called echo sounders or fathometers— ih& more
refined models can even detect schools of fish. All these
methods have in common the emission of a probing
sound, the detection of echoes, and, most important, the
discerning of the distance and direction of the object that
returns the echo.

AIRBORNE ECHOES

The Acoustics of Clicks and Echoes

Because almost every object reflects sound to an ap-
preciable degree, it is very rare for any sound to reach
our ears without embellishment by echoes. Why then
are the echoes so rarely noticed? Seldom do they occur
separately; that is, they rarely arrive at a different time
from the sound that produced them. Usually they and
the original sound are mixed, and we ordinarily fail to
discriminate between the two classes of sound waves.
The simple experiments with a portable radio or tape
recorder suggested in the previous chapter demonstrated
that echoes were present indoors and that they could
make a tone or a noise sound different. The loudness is
increased by the addition of strong echoes from the walls
of a room and by standing waves that may be audible
when continuous pure tones are present indoors. But the
important point is that special experiments were neces-
sary to convince us that echoes really are so common a
part of the most famihar sounds. One major reason
echoes escape our notice so completely is the relatively
long duration of most sounds compared to the time
they require to bounce back in our customary places
of Uving and hstening. Even on the shore of a moun-
tain lake we are not Hkely to notice echoes of the songs
we may sing about a campfire, for they will usually be
masked by the notes that follow. Only when the song
comes to an abrupt ending will the echoes from the hills
intrude upon our consciousness. The masking of echoes
by the continuing sound explains much of our inability
to notice them in ordinary circumstances.

But all sounds come to an end, eventually at least,
and there are always pauses or brief intervals of silence.
Why don't we hear the echoes then? Suppose we try to
investigate the physics of this question by setting up a

ECHOES OF BATS AND MEN

sensitive microphone to convert the energy of sound
waves into electric voltages. Suppose further that we
have connected this microphone to an instrument such
as a cathode-ray oscilloscope, which draws a visible
graph of the sound waves almost instantaneously. A
cathode-ray oscilloscope is the forerunner of your tele-
vision set. Inside the picture tube a spot of Ught is
created, and one electric circuit moves this spot horizon-
tally and again and again at a uniform rate from left to
right while another moves it up and down. In this appli-
cation the up-and-down movement is produced by the
amplified voltage from the microphone. On the picture
screen the combined horizontal and vertical motion lit-
erally draws a graph of sound pressure agamst time.

With such a machine we can watch the behavior of
the sound waves while we utter them. If we suddenly
stop talking, the movement of the spot of Hght on the
picture tube may seem to stop at the same instant. But
if one looks closely, and if the instrument is set up in a
large hall, one can easily see that the oscilloscope con-
tinues briefly to draw a diminishing graph of the sound
waves that are still traveling back and forth past the
microphone from wall to wall. Because sound waves
travel about 344 meters per second, and because less
than 100 per cent of the sound energy is reflected back
from each contact with the walls or floor, and finally be-
cause sound waves are reduced slowly by their frictional
effects on the molecules of the air, the continuing echoes
are appreciable for only a fraction of a second. But they
are there, and our eyes can see them on the oscilloscope
screen even when our ears do seem not to hear them.

With instruments we can improve upon the ability of
our eyes to judge how fast the sound level decUnes and
how long it is detectable at all. One of the simplest
methods is to photograph the moving spot on the oscillo-

AIRBORNE ECHOES

scope screen with a camera in which the fihn moves at
a constant rate. While the spot moves up and down as
the sound waves strike the microphone, the motion of
the fihn draws a graph of sound pressure on the vertical
axis against time on the horizontal axis. The resulting
graphic picture of sound waves makes it easy to see the
echoes which continue to arrive at the microphone a

Fig. 6. A graph of the sound pressure in a very short
word without any echo is shown in A, and the same
word with echoes is shown in B. Note the similarity
of the early waves in A and B, and a difference as the
echo returns before the original word has ended.

good fraction of a second after the end of the sound that
came directly from a speaker's mouth. Such photographs
also show clearly the greater magnitude of the echoes
that follow the same word spoken indoors rather than
out. An example of this comparison is seen in Fig. 6, but

ECHOES OF BATS AND MEN

even here where the word was a short one the echoes be-
gan to mix with the original sound waves long before
the end of the word.

Despite all these differences in the photographic por-
trait, the same word spoken in these two situations
sounds about the same. Again the amazing fact is that
all these special procedures are necessary to convince us
that the two sets of sound waves are not exactly the
same. We have no difficulty recognizing the word or in
telling who said it; it really does not sound very different
in the living room from on the sidewalk. Why not? An-
other simple experiment with a tape recorder throws
considerable light on this question. If we place the tape
recorder in an ordinary room (or, better still, a fairly
large hallway or schoolroom with hard walls) and make
a recording of a short sharp sound, we can play it back
and hear it rather faithfully reproduced. Let us suppose
that such a recording includes several repetitions of
short words ending in hard consonants, such as bit, took,
sud, or leg. Sharp clicks such as one can make by snap-
ping together a pair of large scissors or pUers may also
be used; and if one wishes a good excuse for it, a cap-
pistol report is excellent for this experiment. In any
event each recorded sound should be separated from
the next by a few seconds of quiet.

When such a tape recording has been made, play it
backward. That is, interchange the two reels so that the
tape moves back end first when the machine is playing
back the recording. What used to be the take-up reel
becomes the reel from which the tape is unwound and
vice versa. It will only be necessary on many recorders
to turn the two reels upside down when interchanging
them, so that the same side of the tape will pass next to
the recording head. On some machines only half the
width of the tape is magnetized when a recording is

AIRBORNE ECHOES

made, and when played back in the reverse direction,
this side will not pass by the pick-up head. In this case
the tape must be reversed so that the shiny side rather
than the dull is next to the head. This reduces the level
of the sound, but the volume control can usually be
turned up to compensate for this loss, and the experi-
ment can still be performed, though less well than with
tape which is recorded across its full width.

When the tape is played backward, the echoes that
followed the original word or click will of course pre-
cede it. Since they were hardly noticeable before, one
would naturally expect them to be a faint prelude to the
reversed sound. But the actual result is a startling in-
crease in the apparent loudness of the echoes. A click
that sounded very sharp in its original form, or when a
tape recording of it is played back in the normal direc-
tion, now becomes a gradually rising hiss that culminates
in the cUck. The cUck proper does not sound very dif-
ferent frontward or backward, but the reversed echoes
are much more apparent. So much so that when one hears
this demonstration it is difl&cult to beUeve that the mstru-
ment has not played some trick, that the whooossschk! is
is really the same sound as the sharp cHck that gave rise
to it.

This reversed playback technique reveals the real
magnitude of the echoes from various sounds, but it is
more difi&cult to appreciate with reversed speech or mu-
sic, which sounds very abnormal in other ways. Clicks
or pistol shots are in themselves so short that they con-
tain only a few irregular sound waves, which are not
very different-sounding when played in either direction.
This can be demonstrated by repeating the recording
out of doors in a quiet area well away from any large
building. The clicks will now be accompanied by only
minor echoes from the ground or other small objects,

ECHOES OF BATS AND MEN

such as trees or bushes. When played back in the reverse
direction, they will sound far closer to the original than
they did indoors. In short, this experiment shows the
extent to which our sense of hearing de-emphasizes
echoes. Sound waves which would be clearly audible if
they existed in isolation are almost totally ignored if
they happen to be part of an echo arriving a few tenths
of a second after another sound. This goes far to explain
why spoken words or other sounds do sound nearly the
same when heard indoors with strong echoes from the
walls or out of doors with few echoes or none. Of course
there is a difference if one listens carefully for it, and, in
addition to being louder, speech heard in a closed room
has a "thicker" quaUty. The echoing sound of footsteps
in a very large empty room is a common observation.
Almost everyone has also noted the forlorn sounds of
footsteps or conversation in a house emptied of its fur-
niture and draperies. All these effects are caused by
either the presence or absence of strong echoes.

The mechanism by which we suppress echoes is one
of many subtle mysteries of the human ear and brain,
and no one understands how it is accompUshed. The
suppression lasts only a small fraction of a second; in-
deed, it has been shown to be greatest immediately after
the end of the direct sound and then to diminish grad-
ually until after half a second or so another sound can
be heard about as well as ever. An echo from a distant
hillside arriving four or five seconds after the end of the
outgoing sound is easy enough to hear if it is quiet where
one is listening. But the same strength of echo would be
inaudible if it arrived 1/lOth second after its original
was emitted. By playing a tape recording backward, we
remove the echoes from the time interval when our sup-
pressor mechanism is at work.

In trying to learn what echoes sound hke, it is best to

AIRBORNE ECHOES

use sounds of short duration simply because they are
less likely to overlap and be wholly masked by their
originals. The sounds of spoken syllables or choking
scissors are not as short as one would ideally Uke to use.
Any sound shorter than about 1/lOth second is usually
called a cUck, and the shorter it is the sharper it sounds,
provided it has a reasonably high energy level. The hu-
man voice cannot produce really short cUcks, however,
nor can any other ordinary sound source. An electric
spark caused by the discharge of a condenser is a very
sharp cUck, and the discharge of a condenser through
a loudspeaker is nearly as sharp, provided that the elec-
trical circuit involved does not resonate and prolong the
vibration of the speaker diaphragm. A cheaper and more
widely available source of sharp clicks is a common toy,
the frog or cricket made of a thin strip of spring steel
with a dent in the middle. This is clamped tightly to a
holder at one end; the other end is free to be pushed
back and forth in such a way that the strip is bent and
unbent. When your finger bends the strip of steel, the
dent is suddenly inverted to impart a very sudden and
energetic push to the air as it snaps from concave to
convex or vice versa. The result is a very loud and sharp
click, painfully loud if generated close to the ear, pos-
sibly even damaging if it were to be repeated many times
immediately in front of the ear opening.

The actual duration of the cUck varies from one
model of toy to another, and it is affected by the size
and shape of the holding device. In small clickers that I
have tested the sound falls to 1/lOth its initial maximum
within about 10 miUiseconds after the steel dent has
snapped from one position to another. Recalling that
the velocity of sound in air is approximately one foot
per miUisecond, you can calculate that a click lasting
10 miUiseconds has a physical length of about 10 feet

ECHOES OF BATS AND MEN

as it travels through the air. This means that echoes will
begin to reach the listener after being reflected from a
wall 5 feet away just as the last of the cMck leaves the
device. If we had a cUcker that gave out a 1 -millisecond
sound, this overlap between echo and original sound
would cease at distances greater than 6 inches.

It is interesting to take such a clicker and listen
for its echoes. Even the ordinary toys producing 10-
millisecond chcks will add significantly to the knowl-
edge we obtained with echoes from spoken words. In
these experiments it wiU be important to maximize the
audibihty of the echoes while reducing the level of the
outgoing sound which reaches our ears directly. Part of
the echo-suppressing effect mentioned earlier is a very
brief reduction in the sensitivity of our hearing for a
fraction of a second after the arrival of a very large
sound, and these clickers at close range are really very
loud indeed. A good procedure is to hold a typical toy
clicker with your two hands cupped around it and
opened to form a forward facing horn so that the hands
are between clicker and your ears. The outgoing chck
will still be plainly audible, but its main sound energy
output will be directed straight ahead. All the striking ef-
fects I have described can be heard on reversed playback
of such cUcks. In making a tape recording for reversed
playback, you should keep the microphone behind the
cupped hands, too, so that it also will be better situated
to receive the echoes than to receive the original emitted
click. With this very short click we can also begin to hear
echoes directly without any tape recorder or reversed
playback.

One of these toy clickers held in the cupped hands
can be used to good advantage out of doors. If the
hands and clicker are pointed at a building 50 feet or
so away, a clear and separate echo can easily be heard.

AIRBORNE ECHOES

It can also be used to get distinct echoes from trees a
foot in diameter, and other objects can be located in the
same way. A good technique for a beginner is to sweep
slowly back and forth with the clicker while operating
it at a rate of one or two clicks per second. A few min-
utes of careful Ustening will show that much can be
learned about objects of this general size, provided that
they are at a sufficient distance to yield an echo which
is clearly separate from the emitted cHck itself. Experi-
ence will show that echoes are most easily recognized
when only one large echoing surface is within range.
Several trees in a courtyard surrounded by large build-
ings give multiple echoes that only careful scanning can
resolve.

Before very long your hands become cramped from
the unnatural position in which they must be held in
order both to operate the cUcker and provide it with a
horn. It is not difficult to mount the clicker in a small
horn made of cardboard, Ught metal, or plastic. While
a paraboUc shape is perhaps ideal, a fairly deep cone
will serve fairly weU. The most important point is to
provide a means of bending the dented sheet of steel
back and forth without having any opening at the back
of the horn through which the cUck can reach the user's
ears directly at a high level of intensity. One device of
this sort is shown in Fig. 7.

After you have learned to detect trees and houses by
hearing their echoes, you will find it worth while to ex-
periment with an easily recognized target such as a build-
ing. Keep moving closer as you click. If you find it diffi-
cult to be sure whether you are really hearing echoes, it
may be helpful to try using the chcker while blindfolded
or with your eyes closed. You will then be in much the
same situation as a blind man trying to find his way
about by means of echoes. Many blind people have

TRIGGER IMBEDDED IN RUBBER
TO ALLOW DENTED STRIP OF
STEEL TO BE BENT-

Fig. 7. A very satisfactory device for echo experiments
can be made like this. The inside of the horn should
be a paraboloid of revolution, and the clicker must be
mounted at the focal point of the parabola. The Fiber-
glas and plastic boat- or car-patching materials laid on
a plaster of Paris form make excellent horns, and so
do the parabolic reflectors of certain desk lamps.

AIRBORNE ECHOES

learned to do this with great skill and success. As you
walk toward a building from 15-25 meters away, the
echo of the cUcker is at first clearly separated from the
original cHck but gradually merges with it until there is
only one sound as best you can tell. At this point you
should turn in some other direction, where no large ob-
ject will return echoes, and operate the clicker several
times. The clicks will sound different, and if in doubt
you can alternately point toward the building and then
in some other direction. After this difference has been
recognized, you can move in closer to the building, re-
peatedly clicking both toward it and away in non-echoing
directions. It is surprising how close you can come and
still be clearly aware of a difference in the sound of the
clicker when it is pointed toward and away from the
wall. At very close range, less than 10 feet for example,
the difference will begin to be one of loudness; the
echoes are of sufi&cient intensity that they add apprecia-
bly to the click with which they are fused. This is why
the horn is so important to shield you from the direct
sound; if the horn could be perfect, so that all the sound
energy of the clicker traveled away from you, then the
echoes would become unmistakable.

It is helpful to digress at this point into a little
thought about the wave lengths of audible sounds and
the relationship of these wave lengths to the practicable
size for a horn to direct the click forward. It is a general
property of wave motion that specular (that is, mirror-
like) reflections can be obtained only from objects that
are larger than one wave length. Water waves on the
surface of a ripple tank or a bathtub can be reflected
from the edges of the tank or tub or from objects several
centimeters long. Such reflections obey the same rules
as those that hold for light waves; for instance, the angle
of reflection from a plane surface equals the angle of

ECHOES OF BATS AND MEN

incidence. But quite different results are observed if the
object reflecting the waves is only one wave length or
less. Then one sees secondary waves which may be called
echoes radiating in many directions from the small ob-
ject. The strength of the echo waves in different direc-
tions varies in a complicated way, both with the shape
of the object and particularly with its size, relative to
the wave length. In fact, if the object is much smaller
than one wave length, its shape makes almost no differ-
ence at all. Later on I shall describe some simple experi-
ments with the clicker by which one can see how these
same rules apply to audible sound waves. When the ech-
oes travel in many directions from an object which itself
is small compared to the wave length, they are often
called scattered rather than reflected sound.

But we started this digression to consider how the
wave length of the click would affect the usefulness of a
horn to direct the sound straight forward. A horn is a
special kind of acoustic mirror, and for this purpose we
want one shaped so that sound waves generated some-
where inside will all be reflected from the horn's surfaces,
reinforcing each other and coming out of the mouth
as parallel wave fronts traveling in the same direction.
If the sound is generated at a point, the most effective
horn to concentrate the sound waves into one direction
will be one with a parabolic shape. This means that if
you cut the horn longitudinally, any section will be a
parabola with the sound source at its focus. One of the
geometrical properties of a parabola is that any line
radiating from the focus will strike the surfaces of the
parabola at such an angle that when reflected (at an
angle equal to the angle of incidence) it will be parallel
to the axis of the parabola.

This sounds rather complicated, but perhaps Fig. 8
will help to make it clear. Really this is a very familiar

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Fig. 8. When the wave length is larger than the mouth
of the horn, as in the low-frequency sound waves A^,
Ag, and Ag, there is little or no focusing. But with a
much smaller wave length a narrow beam of plane
waves is produced.

ECHOES OF BATS AND MEN

Story, for searchlights, flashlights, and automobile head-
lights are all made more or less according to this prin-
ciple. But one of the important assumptions we have
made in this line of reasoning is that the sound waves
generated at the focus of a paraboUc horn really would
be reflected from the surface of the horn at an angle
equal to the angle of incidence. This is true only if the
wave length is short compared to the size of the reflecting
surface. K the wave length is much longer than the
dimensions of the horn, very little direction will be im-
parted to the sound waves. This means that a horn must
be several wave lengths in size to do what we want it
to do. What does this tell us about the frequencies of
sound that should be produced by an echo-generating
clicker?

Suppose we decide to use 256 sound waves per sec-
ond. Since the velocity of sound is 344 meters per sec-
ond, this frequency corresponds to a wave length of
344/256, or about 1.3 meters. To be effective, our horn
must be several wave lengths in size, and even if it were
made of the Ughtest possible materials it would be un-
duly bulky. Clearly, then, we want short wave lengths
or high frequencies. But we cannot go to frequencies
above the upper limits of human hearing, which is
somewhere between 15,000 and 20,000 c.p.s. A good
compromise is about 5000 to 10,000 c.p.s. A wave
whose frequency is 10,000 c.p.s. has a wave length
of 344/10,000 meters, or a little under 3 centimeters.
It is quite practicable to build and carry a horn sev-
eral centimeters in size, and if this were the only
consideration we would choose the highest frequencies
or shortest wave lengths that were easily audible. Bats
use frequencies up to 130,000 c.p.s. with wave lengths
down to 2.5 millimeters, and their tiny mouths or ears
can concentrate these short sound waves quite effec-

AIRBORNE ECHOES

tively. The toy clicker produces a number of frequencies
or wave lengths within each brief click, but it would re-
quire much more compUcated click generators to pro-
duce an ideal click containing only a single frequency
and a pulse short enough in duration to yield echoes
distinctly separate from the original. Indeed, this con-
sideration of separateness itself imposes limits on the
possible frequencies. Several waves are necessary to es-
tablish a clear frequency, and if our sound is to last only
1 millisecond it can contain only 10 waves of 10,000
c.p.s., or 5 waves of 5000 c.p.s.

What I have been suggesting in these simple experi-
ments with a clicker is to act as though you were blind
and see what you can discover about the larger objects
in your surroundings solely by means of echoes. Later
on I shall discuss in more detail what blind people
actually do and the success they have achieved as well
as the limitations that seem to prevent echolocation
from warning them about aU the major obstacles that
threaten their safe progress. But before turning to this
direct apphcation to a pressing problem of a large
group of handicapped persons, we will find it helpful to
consider certain physical properties of echoes that de-
termine their strength and audibility. For this purpose
we can make good use of both real echoes from a clicker
and "echoes" in the ripple tank, which is so useful in
the physics laboratory for the analysis of wave motion.

The Velocity of Sound Measured
by Means of Echoes

As a beginning we may consider a simple method of
determining the approximate velocity of sound by an ex-
tension of the already-mentioned procedure of timing the
return of an echo from a distant hillside. If the distance

ECHOES OF BATS AND MEN

to the hill is not known and if the travel time of the
sound and its echo is a few seconds, a good stop watch
(which can measure time to a tenth of a second) would
allow us to determine the distance to the hill, if we as-
sume that we know the velocity of sound. Or if we know
the distance, we can use the same time measurement to
estimate the velocity at which the sound waves travel.
If the basic limit of accuracy in our time measurement
is determined by the stop watch at 0.1 second, the un-
certainty in our measurement of distance would be the
distance over which sound travels in that interval of
time, or approximately 34 meters. But this would be
the round-trip distance, so that theoretically we could
measure the distance to the hill with an accuracy of
±17 meters. Another uncertainty is the human reaction
time, the interval between the actual arrival of a sound
and the pressing of the button on the stop watch. While
this is an appreciable fraction, certainly more than 0.1
of a second, there is not likely to be a great difference
between the first reaction time to the original sound and
the reaction time in stopping the watch when one hears
the echo; hence they will nearly cancel each other. An-
other error is likely to occur if the emitted sound and the
echo build up gradually. If a half second is needed to
reach maximum sound intensity, and if the echo is
enough fainter so that only the peak value is audible,
then we will probably find that the stop watch is pressed
one reaction time after the very beginning of the outgoing
sound, but not until one reaction time after the echo is
nearly at its peak. This can easily cause an error of about
0.3 second unless a very sharp sound is used for the ex-
periment.

A similar experiment can be performed with the
clicker, provided it can be operated fairly rapidly. Sup-
pose you stand 30 meters from a large building and

AIRBORNE ECHOES

point the clicker so that a distinct echo is heard. Since
the sound travels 60 meters from clicker to building and
back to your ears, this trip will require 60/344, or
about 0.17 second. If you operate the clicker twice per
second, you will hear an outgoing chck at a time you
may designate as zero, an echo at 0.17 second, a second
emitted cUck at 0.50 second, a second echo at 0.67 sec-
ond, etc. If we speed up our operation of the clicker,
the second click will eventually come at 0.17 second
and so will mask the echo. If we can operate the clicker
with sufficient regularity, this fusion of echo with second
click provides another way to measure distance— pro-
vided we know the velocity of sound. A mechanical de-
vice such as a metronome can control the rate of click-
ing more precisely, but with a little practice a good
approximation can be achieved. One practical difficulty
is that the click made by bending the strip of steel will
usually be slightly louder or different in quality from
that made when the strip is unbent. Thus successive
clicks alternate in level or quahty, and it is not always
easy to maintain an even rhythm. But it can be done
and, regardless of its practicability, it is worth while to
understand this simple method for estimating distance
by the rate of clicking necessary to cause each echo to
fuse with the following click. One effective way to es-
timate the critical rate is to have someone else count the
number of clicks in a 5- or 10-second period measured
with a stop watch or the second hand of an ordinary
watch.

The same cUcker may also be used to demonstrate
convincingly the concentration of echoes into certain
directions when they have been reflected from surfaces
of various sizes relative to the wave lengths in the click.
Most toy clickers have a frequency range between 3 and
10 kilocycles, so that the most intense soimd waves have

ECHOES OF BATS AND MEN

wave lengths of a few centimeters. When such wave
lengths strike the wall of a building, they are reflected
almost exactly as Hght waves would be from a mirror. If
the clicker is pointed directly at the wall, the echo will
come straight back, but if the emitted sound strikes ob-

Zdab= Zeac

Fig. 9. The law of reflection describes the way in
which sound reflects from a large flat surface. When
making this experiment, observe the relative positions
of the two boys.

liquely, the echoes will rebound away from the clicker,
as indicated in Fig. 9. This is why it is so easy to locate
a building by scanning with the clicker; the echo is far
louder when the horn is pointed straight at the wall. Two
people can co-operate in a simple experiment that dem-

AIRBORNE ECHOES

onstrates how these echoes behave. One should aim the
clicker at the wall 20° to 30° to one side of a perpen-
dicular from clicker to wall, while the second listens for
the echo. He will not hear it so clearly if he stands be-
side the clicker at point B as he will if he walks to one
side and a Uttle behind the clicker to a point such as
C. The position where the echo is loudest can be pre-
dicted on the same principle that governs the specular
reflection of light from mirrors; namely, that the angle of
reflection, r (angle EAC), equals the angle of incidence,
/ (angle DAB). This experiment will give clearer results
if the listener stands a little behind the clicker, so that
he is shielded from the direct, outgoing click by the horn.
The same experiment can be performed more accurately
by mounting the clicker on a camera tripod and turning
it slowly to different angles relative to the wall. The lis-
tener may then move back and forth until he finds the
points where he hears the echo most clearly. Or the Us-
tener may stand still in various positions while the first
person turns the clicker slowly back and forth from right
to left according to his instructions. It is remarkable how
well the results of such experiments confirm the rule that
the angle of reflection equals the angle of incidence.

There is an entirely different situation in which it is
easy to experience a simple type of echolocation. When
you ride in an automobile, sitting by an open window,
you hear a number of soimds from the engine, the tires,
and the rush of air past the window. As you drive past
a high stone wall, through an underpass, or close to any
large surface, these sounds will change in quality. A se-
ries of concrete guardrail posts, the masonry posts used
to support iron fences, or even a row of wooden fence
posts can be detected from a rapid series of swishing
sounds as the car moves by. Try Ustening with your eyes
closed as you ride along some familiar route and you

ECHOES OF BATS AND MEN

may be surprised to find how many places you can rec-
ognize by ear. If you find a series of clearly "audible"
fence posts, compare their sound effects with those you
hear in passing through an underpass. Along the posts
it is primarily the high frequencies that return as echoes
from the relatively small surfaces; in the underpass al-
most the whole range of sounds of the car will be re-
flected from the large wall surface. K you make a care-
ful study of these sounds while your car is driven at
about the same speed, you will find that you can learn
to recognize many types of structures, such as parked
cars, from the echoes which they add to the roughly con-
stant sounds made by your own car.

Echoes are used by bats and men to locate smaller
and more elusive objects than the walls of buildings,
and some interesting properties of reflected waves be-
come important once we begin to work with smaller ob-
jects. After you have acquired some experience with the
chcker, it is of interest to try it on trees, telephone poles,
or other objects that can easily be found out in the open
away from other echo-making objects. With care and
practice you can detect trees as small as 6 inches from
several feet away, and when this has been accomphshed,
you can again call upon another person to point the
clicker at the tree while you, the listening observer, move
about to different positions to find where the echo sounds
loudest. The result will usually be that the echo can be
heard over a much wider range of angles than hap-
pened with the louder echo from a building. This is
because the tree is only a Httle more than one wave
length in diameter and the echoes are spread over a
much wider range of directions, as indicated in Fig. 10.
Just as a horn less than one wave length in size fails to
concentrate sound, small objects scatter their echoes.
If you can hear echoes from trees or poles as small

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and the width of the lines the intensity of sound.

ECHOES OF BATS AND MEN

as one or two wave lengths, you will find them al-
most equally loud over a wide angular range. Of
course they are nowhere as loud as those from larger
structures such as buildings. This would be just as
true of light waves or water waves, and an appropriate
experiment in the ripple tank will show specular reflec-
tion of surface waves from long objects but would show
extensive scattering from something about one wave
length in size.

This difference between specular reflection and scat-
tering of waves can be studied with a ripple tank or even
with the surface waves in a bathtub, although it is more
difficult to see them clearly in the tub. Just as echoes are
easier to hear if generated by sounds of short duration,
it is easier to study surface echoes by generating short
trains or pulses of water waves. This is probably why
water beetles interrupt their swimming motions at fre-
quent intervals— to provide intervals of "quiet" in which
they can better feel the echoes from objects at some dis-
tance across the water's surface. If one sets up a few sur-
face waves at a time by a quick light tap against the
water, reflections from the edge of the tank or tub are
of course easy to see. If an object of about one wave
length (for example, a short piece of broomstick or
wooden dowel) is placed in the water with its axis per-
pendicular to the surface, close observation will find
smaller waves scattering out in almost all directions from
this source of surface echoes. Of course all other waves
must be absent, but, once this phenomenon has been ob-
served, it is of some interest to vary the size of the cy-
lindrical object from the smallest that produces visible
scattered waves up to sizes well in excess of one wave
length. Such experiments convince one of the real dif-
ference between sharply directional specular reflection
and the diffuse scattering from small echo sources. These

AIRBORNE ECHOES

two general types of echoes will be important when we
move on from the physics of echoes to a study of the
actual uses to which they are put by blind men and by
the bats and other animals which have developed such
refined and precise methods of echolocation for the
carrying out of their daily business.

CHAPTER 4
The Language of Echoes

From our brief, qualitative look at the remarkable navi-
gational feats of some animals, it seemed clear that sound
was a most important message carrier. This led us to a
detailed examination of sound itself, particularly how it
echoes or reflects, in order that we might experiment
more skillfully and intelligently in an attempt to learn
how echoes are actually used by animals— what are their
limits, what aids or hinders, what physical characteristics
are especially suited to echolocation, what are the special
characteristics of the sounds these animals make. We
may hope to discover some important bits of evidence,
perhaps obscure at the moment, which will aid blind peo-
ple in their travels, and even if this does not occur, we
will certainly know our environment better. Men have al-
ways learned from animals, and even in this age of elec-
tronics and atomic structure we still have much to learn.
Since the bats are so expert in the use of echoes, let us be-
gin by examining in more detail the sounds they broad-
cast to produce the echoes which guide their agile flight.

ECHOES OF BATS AND MEN

Orientation Sounds of Bats

Bats make a variety of vocal sounds; for example,
when disturbed they squeak and chitter. But we are in-
terested primarily in the sounds they use in flight to gen-
erate useful echoes that tell them about objects at some
distance. These orientation sounds are all of high fre-
quency, though they overlap slightly the range of human
hearing to produce the very faint audible ticking we have
discussed. But most of the sound energy emitted by fly-
ing bats lies at frequencies from 10 to 150 kc in different
species, and I will describe only one or two examples
of orientation sounds that have been measured from a
few typical kinds of bats.

One of the simplest acoustic patterns is that used by
the horseshoe bats, an insectivorous group that lives in
Europe, Asia, Austraha, and Africa. They use orienta-
tion sounds of nearly a single frequency, which may be
anywhere from 60 to 120 kc, depending on the species.
The individual sounds last only a small fraction of a
second, usually from 50 to 100 miUiseconds, but this is
much longer than the duration of other bats' sounds.
The name horseshoe bat refers to a complicated se-
ries of folds or membranes surrounding the nostrils and
the mouth with two roughly concentric rosettes which
vaguely resemble a horseshoe when viewed from in
front. The German zoologist Franz P. Moehres has
shown recently that the horseshoe serves as a small horn
to concentrate the emitted sound into a narrow beam
which is swept back and forth as the bat scans its sur-
roundings. Bats have a habit of hanging head downward
by the hind feet, and the horseshoe bats have especially
flexible hip joints. They can pivot through almost a
complete circle as they scan with their beam of high-

THE LANGUAGE OF ECHOES

frequency sound. Often they dart out from such a posi-
tion to seize an insect that flies within range.

Another group of bats, confined to the tropics, feed
mostly upon fruit, but some also eat insects, which
they may pick off the vegetation. These bats emit
much fainter sounds than the horseshoe bats— ex-
tremely brief clicks, lasting from a fraction of a milU-
second to 2 or 3 milliseconds. The sound waves making
up these very short pulses are compUcated, with a va-
riety of frequencies from as low as 10 to as high as 150
kc, again depending on the species. The vampire bats,
which feed on the blood of Uving animals and men, be-
long to this group. Without causing the victim to awake
from his sleep, they feed by making small cuts with their
very sharp teeth and drinking the blood that flows for a
time before clotting. All these bats seem to refrain from
the active pursuit of flying insects, and the intensity level
of their sounds is so low that only the best of micro-
phones and sound-analyzing equipment will register
them. They may be called whispering bats to distinguish
them from the other two groups.

The third major category includes the common in-
sectivorous bats that are well known in North America
and Europe. With a very few exceptions, these bats all
hunt flying insects in the open, tracking their elusive
moving prey on the wing, maneuvering through com-
plicated spUt-second turns and other acrobatics to follow
and intercept the erratic flight of moths and flying
beetles, May flies and mosquitoes. The sounds used by
this group, only a few miUiseconds in duration and al-
most as intense as those of the horseshoe bats, have a
characteristic frequency pattern. Each orientation soimd
starts at a very high frequency and drops rapidly during
its brief life, to end about an octave below the frequency
at which it started. The common little brown bats of the

ECHOES OF BATS AND MEN

Fig. 11. The frequency and wave length of a bat's
sound vary during each chirp. This diagram, which is
approximately to scale, illustrates the small amount of
sound reflected by one insect.

United States begin each of their orientation sounds at
about 90 kc and end at 45 kc. Since each sound lasts
only about 2 milliseconds, this is a very rapid change in
frequency; indeed, this bat sweeps through a frequency
band double the whole range of the human ear in 2

THE LANGUAGE OF ECHOES

milliseconds. As illustrated in Fig. 11, a typical orienta-
tion sound contains only about 50 sound waves, no
two exactly alike. The wave length of the initial waves
is only half the wave length of those making up the end
of the emitted sound. These sounds are chirps, at least
that is what we call audible sounds made by certain in-
sects when they sweep through as wide a range of fre-
quencies within a fraction of a second. This type is
sometimes called a frequency-modulated pulse of sound,
and this group of bats may be thought of as "FM bats"
in contrast to the horseshoe bats with their much longer,
sharply beamed tones of nearly constant frequency, and
the faint but complex cHcks of the tropical "whispering
bats."

Echoes of Insect Prey

The FM or chirping bats have been studied much
more thoroughly than the other two groups; therefore,
more is known about them. They seem to be the most
highly specialized for a life of flight, very expert at ma-
neuvering under the most difl&cult conditions. The daily
(or nightly) business of catching insect food compels
them to be highly skilled in the detection of such small
moving objects and in the aerial acrobatics necessary to
intercept them. Since bats do almost all their hunting on
dark nights, often approaching insects from above or in
wooded areas where they would have to be seen against
a dark background, visual detection must be impossible.
And SpaUanzani, as we have said in Chapter 1, showed
before 1800 that blind bats catch as many insects as
normal animals. It has usually been thought that they
located insects by listening for the sounds of their wing-
beats, and this probably does occur in some circum-
stances when the flying insects make appreciable hum-

ECHOES OF BATS AND MEN

ming or buzzing noises. But I have discovered in recent
years that the orientation sounds, the high-frequency
chirps of these bats, are repeated at remarkably high
rates as the bats locate and close in upon flying insects.
Furthermore, bats will often pursue imitation insects
such as pebbles or Uttle wads of wet absorbent cotton
tossed gently into the air as they fly by. They do not actu-
ally bite or swallow such decoys, but they swoop avidly
towards them with the same increase in the tempo of the
orientation sounds they employ when chasing real in-
sects under natural conditions. When one realizes how
silent are many of the small insects upon which bats
feed, it becomes rather Ukely, though not rigorously
proven, that the bats detect at least some of their insect
prey by hearing echoes of their own chirps bouncing oQ
the insects rather than relying solely on the sounds
emitted by the insects themselves.

I shall return a Uttle later to the patterns in which
these orientation sounds are broadcast under various
conditions, including the pursuit of insect prey. But first
let us consider the effectiveness of the process of insect
himting. Just how many insects does a bat catch in a
given time? How big are the insects caught? At what
distances are they detected? Only very recently have we
been able to provide even partial and tentative answers
to such questions. Spallanzani and others who examined
the stomachs of bats just returned from a night's hunting
have marveled at the relatively large mass of finely
chewed insect remains present in the digestive tract of
every successful bat. One study showed that little brown
bats weighing 7 grams commonly catch 1 gram of insects
per hour of active hunting. Very recently we have been
able to persuade a few bats to hunt insects in a laboratory
flight room where the process could be studied and pho-
tographed. One smaller relative of the little brown bat,

THE LANGUAGE OF ECHOES

weighing only 3.5 grams, caught mosquitoes at such a
rapid rate before our very eyes that after 15 minutes'
hunting its weight had increased by 10 per cent to 3.85
grams. These particular mosquitoes weighed about
0.002 gram each. The bat had no possible way of gain-
ing weight during these 15 minutes of closely observed
himting, aside from the weight of the mosquitoes caught.
It drank no water and ate nothing else. It probably lost
a little weight by the evaporation of water while breath-
ing; therefore, it caught more than 0.35 gram of mos-
quitoes.

Dividing the weight gain by the weight of a single
mosquito shows that at least 175 mosquitoes were
caught in 15 minutes, or more than one every 6 seconds.
This was also approximately the number of obvious
mosquito-chasing maneuvers that we could count dur-
ing this hunting spree. There is every reason to believe
that similar rates of insect capture are commonplace
events in the nightly activities of millions of these bats
and their relatives all over the world. Of course, it is not
always mosquitoes that are eaten; almost any kind of
insect that is locally available and is not too big seems
welcome. Sometimes moths up to an inch in wingspread
are taken, but at other times these bats capture insects
much smaller than mosquitoes. In one instance a smaU
gnat weighing only 0.0002 gram was found stiQ unswal-
lowed in the mouth of a bat killed while it was hunting.

This maneuvering to capture one insect every 6 sec-
onds is what makes the flight of bats appear so erratic.
Far from being feeble fliers buffeted about by air cur-
rents, they are expert fliers engaging in the difficult in-
terception of flying insects. Their percentage of successes
must be very high indeed. Certainly they are doing vastly
better than simply flying around with their mouths open.
Even when mosquitoes are particularly abundant, their

ECHOES OF BATS AND MEN

density is such that one of these small bats would have
to fly all night before its mouth encountered a single
mosquito purely by chance. Yet the actual rate of capture
is one every few seconds.

Photographs of the bat pursuing an insect show that
they sometimes begin their maneuvers when 2 or 3 feet
from a mosquito. The pattern of the orientation sounds
begins to change a fraction of a second before the bat
turns toward its victim. The implications of these obser-
vations can be understood after a brief explanation of
the rate at which a bat's frequency-modulated chirps are
repeated during various types of flight. When a Uttle
brown bat is flying fairly straight and is not close to any-
thing of immediate concern, it repeats its 1- to 2-milli-
second chirps at rates of 10 to 20 per second. But when-
ever it approaches any small obstacles, such as wires
stretched across a laboratory room to test its skill, many
more chirps are emitted in a given interval of time. For
brief periods the repetition rate may rise as high as 250
per second. When this happens, the individual chirps be-
come shorter, usually less than 1 millisecond, so that
silent intervals still exist between chirps. When the high-
frequency sounds of these bats are studied under natural
conditions, a clear distinction between straight and level
flight and the active pursuit of flying insects becomes ap-
parent. Such eavesdropping is only possible, of course,
when we have apparatus which will detect the bat sounds
that are inaudible. In one convenient form this apparatus
"translates" each of the bat's high-frequency sounds into
audible clicks in earphones or a small loudspeaker. This
makes it possible to watch the bat while at the same
time "listening" to its orientation sounds in this trans-
posed form. Most of the details, such as the octave of
frequency modulation, are lost, but there is one cUck

THE LANGUAGE OF ECHOES

from the loudspeaker every time the bat broadcasts one
of its high-frequency chirps.

When this "listening" apparatus is used in some spot
where bats do their insect hunting, we notice that one
cruising past on a straight course several feet above the
ground sounds like the slow putt-putt-putt-putt of a lazy,
old gasoline engine. Often it will fly straight past with
Uttle or no change in this rhythm, but if its attention is
attracted either to a real insect or to a decoy, such as a
pebble tossed up in front of it, then there is a marked
increase in the rate of the orientation sounds. Sometimes
this is a slight increase in rate, but if the pursuit is serious,
involving drastic maneuvers such as sudden turns, wing-
overs, or sharp dives, then the translation resembles the
acceleration of a motorcycle engine. On occasion it rises
to a real crescendo with the individual clicks coming so
close together that for human ears they fuse into a whin-
ing buzz reminiscent of a chain saw. Such crescendos
occur just when the bat is closing in on an elusive moving
target, strong evidence against the idea that all location
and tracking are done simply by listening to the sounds
of the insect's wingbeats. In this case one would expect
the bat to keep relatively quiet when near an insect so
as to hear the faint buzzing of the insect's wings. Instead
it fills the air with an extremely rapid series of chirps
that would seem to interfere severely with any process of
passive Ustening.

Precision of Echolocation

Another important aspect of bats' use of echoes for
rapid and precise navigation is the small size of objects
which can be detected and the distances at which detec-
tion can occur. The only feasible tests yet devised have
involved wires or strings rather than small isolated ob-

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THE LANGUAGE OF ECHOES

jects like insects or pebbles. It is simply too difficult to
keep small particles stationary in the air long enough to
make accurate tests of bats' ability to dodge them. When
wires are strung across a laboratory "flight" room, on
the other hand, as diagramed in Fig. 12, the animals
seem anxious to avoid collisions, although the brown
bats weigh so little that they do not seem to be injured
even in an occasional head-on crash against a taut wire.

When the wires are spaced 30 centimeters apart, or
sUghtly more than the wingspread of the Uttle brown bat,
they make a difficult barrier that even the most skillful
animals brush against lightly from time to time. The
wires can be made smaller and smaller, without any
marked effect on the percentage of misses registered by a
really skillful animal, down to a wire diameter of a frac-
tion of a millimeter. To be sure, many bats wlQ be found
on first testing in such an obstacle course to be clumsy,
striking even the larger wires, but this is usually because
they are in poor condition or not completely awakened
from the deep sleep into which they lapse even on sum-
mer days. It is necessary to reduce the wire diameter to
0.07 millimeter (about the diameter of a human hair)
before the little brown bats strike them at random. Even
slightly larger wires, 0.12 millimeters in diameter, while
difficult to miss, are dodged by the really skillful "ath-
letes" among our experimental subjects far more often
than one would expect.

Astonishing as it is that bats can detect wires as small
as 0.12 millimeters, these previous experiments do not
tell us at what distance this detection occurs. But motion
pictures of the bats will give some indication of the range
of detection when the translated orientation sounds are
put on a sound track of the movie. Careful study of such
movies, frame by frame, has enabled us to find the dis-
tance at which the rate of repetition of the bats' chirps

ECHOES OF BATS AND MEN

first begins to increase. This varies from flight to flight,
even for a single individual, but the average of numerous
measurements with several sizes of wire gave the follow-
ing results.

Average

rep. rate

distance

Diameter

before

Average

at which rep.

of wires

approach

maximum

rate first

(miUi-

to wires

rep. rate

increases

meters)

(pulses/sec)

(centimeters)

3.0

215

1.07

185

0.65

150

0.46

120

0.28

105

0.18

These distances are considerably greater than one
would guess from the bat's flight behavior. Ordinarily
it flies along a fairly straight course and swerves only in
the last few inches to avoid a wire. Yet the increasing
pulse rate shows that it has already detected the wires
and reacted to them at the distances shown in the table.
If no wire is in place, there is no increase in the rate of
the orientation chirps. Of course, a bat might be aware of
the wires at still greater distances than the table shows,
but it gives no sign of such awareness that we yet have
learned to recognize. The important point is that even
such small wires as those 0.18 millimeters in diameter
are detected at some distance, not merely at the last pos-
sible moment to avoid colUsion. It is also interesting to
note that small wires produce only a small increase in
pulse rate. Actually the bat is moving so fast (ap-

THE LANGUAGE OF ECHOES

proximately 4 meters per second) that with the smaller
wires it often has time for only 2 or 3 additional pulses
above the number it would have employed had there
been no wire in place. All these facts testify that the
echolocation practiced by bats is a refined, accurate
method of orientation, not merely a crude sort of
gropmg.

Bread upon the Waters

Nor do these examples by any means exhaust the list
of difficult tasks which bats accomplish with some aid
at least from echolocation. Certain of the whispering bats
catch insects, small birds, or lizards that are resting on
vegetation, but we are not sure that they do this by means
of echolocation. They may simply listen to characteristic
sounds coming from their prey. More amazing is the fact
that four different species of the FM bats make their liv-
ing by catching fish. This they do by flying just above the
surface of the water and every now and then dipping
their hind feet just below the surface. The claws on these
feet are long, curved, and sharp, and the bats manage
to gaff small minnows often enough to fill their stomachs
every evening (as shown in Fig. 13 ) . When fishing in this
way on the darkest nights (and often with mist rising
from the water) , they emit a rapid series of chirps much
like those of their insect-catching relatives. The gap be-
tween the two types of food gathering is not as great
as it might at first seem, for the insectivorous bats drink
by skimming the surface of the water and dipping their
chins just deep enough to secure a drop of water at a
time. This requires fine control, for a millimeter too deep
would surely result in a dunking. These insectivorous
bats also catch insects resting on the water surface, so
perhaps it was a small step from this habit to reach for

ECHOES OF BATS AND MEN

fish below the surface. In any event, the fish-catching
species make much of their living in this way, and during
their recent evolutionary history a relatively small ana-
tomical adaptation has resulted in the specialized fish-
gafiSng claws.

When I have watched these bats in Panama, I have
seen no sign that the fish were moving or disturbing the
surface of the water in any way. Often it was glassy
calm, and the bat flew for hundreds of feet a few inches
above the surface, quickly lowering the hind feet into the
water for a short distance and then raising them while
continuing its low-altitude searching flight. How do these
bats know where fish are to be captured? They are evi-

Fig. 13. Motion pictures of fishing bats actually gaf-
fing minnows provided the model for this drawing.
Prentice Bloedel took the photographs.

dently selective in their fishing, for they fly long distances
just above the water and only rarely dip their claws be-
neath the surface. Since the fishing occurs on dark and
misty nights, it is most unlikely that the fish could be
seen and still less probable that they emit any sound
audible to the bat flying in the air above the surface.
Could it be that the fish-catching bats detect echoes from
fish beneath the surface? At first glance this may seem
only a slight modification of the process by which closely
related bats catch insects in the air. But the physical dis-
continuity between air and water makes the transmission

THE LANGUAGE OF ECHOES

of sound difficult, and so echolocation seems an unlikely
explanation.

As mentioned earlier in connection with the underwa-
ter hearing of fish and porpoises, sound waves have great
difficulty in passing from air to water or vice versa. When
airborne sound impinges on a smooth surface of water,
with its direction of travel perpendicular to the water
surface, only 0.12 per cent of the energy of the airborne
sound continues beneath the surface as underwater
sound waves. For a sound wave travehng from water to
air, the same small fraction of the acoustic energy strik-
ing the surface from below continues outwards into the
air. This means that a flying bat's orientation sounds
striking the water, penetrating into it, being reflected
back from a fish, and passing out into the air again
would be reduced to (0.0012)2, or 1.44 x 10"^ of the
original sound, during the two trips through the air-
water interface. To this great reduction must be added
further losses: only a small fraction of the emitted
sound would be reflected by a fish, and only a small
fraction of what did escape into the air would strike
the ears of the listening bat. These figures make it
seem almost hopeless for a bat to try to detect fish
through the water surface by their echoes, but before
dismissing the whole idea as utterly impossible, let us
compare what insect-eating bats are known to do in air
with the hypothetical location of fish by their echoes.

Certain of the FM bats are able to detect a pebble or
a flying insect 1 centimeter in diameter from at least 200
centimeters away. At distances of more than about 10
centimeters from the bat's mouth the sound intensity falls
off as the square of the distance. Since a 1 -centimeter
insect is a small target, sound is scattered from it ap-
proximately as it would be from a point source, so that
the echo intensity also varies inversely as the square of

ECHOES OF BATS AND MEN

the distance. Therefore, as the distance from bat to insect
increases, the net strength of the echo returning to the
animal's ears falls off as the fourth power of the distance.
If the insect is twice as far away, the intensity of its echo
is 1/2*, or l/16th as great. Let us suppose, for the
sake of argument, that a fishing bat does detect a small
fish at a distance of 10 centimeters. Since a fish's body is
acoustically similar to water, any echo it produces would
be most likely to come from the swim bladder. This is an
air-filled chamber which most small fresh-water fish have
within their bodies, and in a minnow it would be about
1 centimeter, or the same size as the insects detected in
air at 200 centimeters. If all other factors were equal, a
target at 10 centimeters would return an echo stronger
than one at 200 centimeters by a factor of (200/10)*,
or 1.6 X 10^. The two trips through the water surface
would reduce the fish echo by a factor of 1.33 X 10"^.
The product of these two numbers is 0.23, which would
mean that the echo received by the fishing bat under
these hypothetical conditions would have about one
fourth the intensity of the echo which is actually detected
by the bat catching insects in air. If this is really a vaHd
comparison, it puts the possibility of catching fish by
echoes in quite another light, since a factor of four is well
within the uncertainty of the assumptions I have made.
For example, the insect-catching bat may well detect
1 -centimeter insects at more than 200 centimeters, and
an increase in the distance of detection to 280 centi-
meters would produce an echo from the insect equal to
that from the hypothetical fish echo.

This numerical argument, however, does not prove
that the fishing bats really do hear echoes from fish
through the water surface. It simply means that this pos-
sibility merits consideration and should not be rejected
out of hand because of the very large energy loss during

THE LANGUAGE OF ECHOES

the two passages through the air-water interface. Or, to
put the matter in another way, the detection of insects
at 2 meters through the air means that bats are capable
of hearing echoes roughly as faint as those that might,
under ideal conditions, return from a minnow to a fish-
ing bat. The book Listening in the Dark goes into this
particular problem in more detail if you wish to pursue
it further. But it is significant that a hypothesis which
seemed so completely ridiculous when one first learned
of the milUonfold loss of energy during the round trip
from air to water should turn out, on closer examina-
tion, to be a serious possibility after all. Common sense
and first impressions may be misleading when we are
dealing with matters quite outside the range of ordinary
human experience upon which people have built what
we call common sense.

Resistance to Jamming

Up to this point we have been thinking about echoes
as more or less isolated sound waves that could be dealt
with one at a time. To be sure, we considered earHer the
likeUhood that a faint echo would be masked by the
louder outgoing sound. Experiments described in Chap-
ter 3 demonstrated that human hearing ignores echoes
arriving within a small fraction of a second after a loud
sharp chck. Bats obviously do better than we in dis-
criminating these echoes from the original sound. In the
experiments of Schevill and Lawrence a porpoise showed
that it could detect echoes from a small fish despite the
louder competing echoes from the bottom of the pond,
the surface, and the shore a few feet beyond this small-
sized target. But the expertness of bats goes even further
than anything we have yet considered. When they are
hunting insects, their ears receive a more complicated

ECHOES OF BATS AND MEN

mixture of sounds than merely their original chirp plus
a single echo returning from a single insect and having
the same wave form at a lower energy level. What really
impinges upon their ears is a whole series of echoes from
everything within several feet— the ground, other insects,
and every bush, twig, tree trunk, leaf, or blade of grass.
Many of these things contribute only small amounts of
echo energy, but the echo from an insect is itself a faint
one, and if it is audible so must the others be. How then
do bats sort out one class of faint echoes from all the
others? How do they hear the difference between echoes
that mean food to be caught and those that mean ob-
stacles to be dodged?

If we knew how bats discriminate so expertly be-
tween faint insect echoes and the competing echoes ar-
riving within a small fraction of a second, we could make
more rapid progress toward solving the orientation
problems of blind people, to say nothing of developing
instruments that could emulate the bats more perfectly.
Unfortunately this is not yet possible, but it is interesting
to consider how well bats can make such discrimina-
tions. This cannot be easy, even for a bat, and faint
echoes from wire obstacles are less skillfully utilized
when stronger echoes arrive in the same small fraction
of a second. For example, we once performed an ex-
periment in which two rows of wires were stretched
across a flight room, one row at the middle of the room
and the other row 45 centimeters from the end wall, as
diagramed in Fig. 12. In both rows the wires extended
from floor to ceiling and were spaced 30 centimeters
apart. When the diameter of the wires was 0.46 milli-
meter, they were difficult echo targets, but the percent-
age of misses in a large number of flights through the
central row was 91 per cent. This represents a considera-
ble degree of success, and almost all the contacts were

100

THE LANGUAGE OF ECHOES

very light touches when the bat did not quite manage
to time its wingbeats so that the wing tips cleared the
obstacle.

When the same animals not only flew through the mid-
dle row but also continued on through the end row, their
success was much less— the percentage of misses fell from
91 per cent at the middle row to 58 per cent for the end
row which was 45 centimeters from the end wall. This re-
sult was probably due to the very much larger echo from
the end wall. The situation can be understood in terms of
Fig. 14, a schematic graph of the sound energy reaching
the bat's ears during the fraction of a second when each
chirp is emitted and its several echoes return. The upper
graph (A) depicts the situation when the middle row of
wires is being detected and avoided; the middle graph
(B) applies to the same size of wire located 45 centi-
meters from the end wall, while the third graph (C) de-
scribes a further experiment in which the wires near the
end wall were 1.07 millimeters in diameter. In C, the
bat's success was about the same (88 per cent misses)
as it had been with the 0.46 millimeter wires at the mid-
dle of the flight room ( A) . Under natural conditions the
important echoes would be those from an insect rather
than a wire, and the competing echoes would arrive from
many different objects, such as the ground, tree trunks,
or branches of trees. These would produce more com-
pHcated echoes than those from the end wall of the flight
room and would be present over a longer period of time,
but they would never include as strong a single echo as
that from the large end wall. An approximation to this
case is represented in the fourth graph (D) of Fig. 14,
where it has been assumed that some of the extraneous
echoes have come from objects closer than the insect it-
self. This must happen when bats hunt, as they often do,

101

BAT'S ORIGINAL PULSE
P

END WALL ECHO

WIRE ECHO
W

m —

fi^^tA^l^^^

Fig. 14. Schematic graphs of a bat's chirp and the
echoes in the flight room {Fig. 12) under various test
conditions: A— approaching 0.46-millimeter wires in the
middle of room; B— approaching same wires near the
end wall; C— approaching larger wires close to the end
wall {note the larger echo); and D— approaching an
insect under natural conditions in woods where many
other objects also return echoes. {For simplicity the
frequency modulation is not shown.)

102

THE LANGUAGE OF ECHOES

in thickly wooded areas where competing echoes ob-
scure the important echo from the insect.

The success of bats in catching one insect every few
seconds testifies to their ability not only to hear the in-
sect echoes but to sort them out of a welter of other,
competing echoes. This process has been studied in the
laboratory by modifying the circumstances to standard-
ize the conditions and permit measurements of the bats'
performance. Rather than studying bats as they hunt
insects in the woods, we generated artificial sounds in
our flight room so that these noises were added to the
echoes from wires, floor, and walls. In other words, we
tried to confuse or "jam" the bats. The result was a sur-
prising and revealing failure. The bats continued to
dodge wires of 1 to 2 millimeters even in the most in-
tense noise we could produce, a loud hissing that covered
the whole frequency range of their orientation sounds.
Skillful animals avoided wires of this size just as well
in the noise as in the quiet, even though the noise was
much louder than the echoes from the wires. These ex-
periments could theoretically be shown as a fifth graph
in Fig. 14, but an accurate representation of the noise
would obUterate any representation of echoes from the
wires.

There are limits, however, to the discriminating ability
of even the most skillful bat. If the wires are made
smaller and smaller, a size is finally reached where the
echoes no longer can be detected. The smallest wire
which can be detected in noise is greater than the small-
est wire detectable in quiet. For one species of bat the
minimum in the quiet was about 0.25 millimeter, and
in the noise the minimum size increased to 0.5 to 0.7
millimeter, depending upon the individual animal and
its condition at the time of the particular experiment.

103

ECHOES OF BATS AND MEN

(Listening in the Dark has a more detailed account of
these experiments.)

What emerges from these several examples of orien-
tation based on echoes is the simple fact that bats and
porpoises are most adept at locating small and distant
objects in this way. Furthermore, they do so with a pre-
cision and acuity that are understandable only when one
remembers that this is how they make their Uving. If a
bat fumbles with its echoes, it goes hungry. Hunger is
a powerful incentive, tending strongly to improve any
mechanism or process subjected to this selective action.
This is what biologists call natural selection, the process
responsible for the evolution of plants and animals into
their many diversified and complex forms. It is a slow
process but an extremely effective one, and in the bats
and porpoises we see the end result achieved through
natural selection, perfecting over miUions of years the
animals' faculties for utilizing echoes. Finally, it is im-
portant to realize that the use of echoes requires the bats
and porpoises to possess more than merely a means for
generating sounds that in turn will yield echoes. It is also
essential that these animals discriminate certain impor-
tant echoes from complex mixtures of other sounds that
are often much louder than those conveying the crucial
information about food.

Discrimination of one portion of a complex sound
from louder components is not a special skill of bats
and porpoises. All animals endowed with a sense of
hearing discriminate, and in many respects the human
ear and brain are the best of all. When we listen to
speech or music, we sort out a few significant portions
of a compUcated mixture of shifting wave forms. If we
hear people speaking an unknown foreign language, we
receive a similar jumble of sound waves, but one to
which we have no key. Footsteps or bat chirps and their

104

THE LANGUAGE OF ECHOES

echoes are a special language of their own. It is much
simpler than German, Chinese, or English, but men,
particularly blind men, find it very difficult to learn this
language. Yet bats no larger than a baby mouse under-
stand it well enough to catch ten mosquitoes every min-
ute in the dark. What is it in a bat's tiny brain that per-
mits understanding of this language and unlocks this
library of useful information? No one yet knows the
answer. We cannot even be sure we are asking the
proper questions.

105

CHAPTER 5
Sonar and Radar

Although men have not learned the "language of
echoes," they have been remarkably successful in de-
signing echolocating instruments which surpass those
of animals in many ways but remain quite inferior in
other respects. What are these instruments and how do
they compare with analogous living mechanisms in the
bodies of bats, porpoises, or whirligig beetles? Footsteps
and clickers are simple devices that help blind people
create more useful echoes, but the receiving instrument
is still the human ear. Perhaps blind men will some day
learn to exploit the potentialities of the matchless human
brain for a better comprehension of the language of
echoes. But, in the meantime, it is important to appre-
ciate the devices which men have contrived to carry out
both the sending and the receiving functions of echoloca-
tion. These mechanisms have been developed for very
practical, often military, purposes, excelling particularly
in the great distances over which they operate. If they
utilize sound waves, they are usually called sonar sys-
tems. If electromagnetic waves are employed, they are
called radar systems. Sonar is used by man almost ex-

107

ECHOES OF BATS AND MEN

clusively for underwater echolocation, while radar is
used only in air or outer space.

Echoes under Water

The tragic sinking of the Titanic by an iceberg in 1912
prompted the first of many efforts to invent a means of
detecting icebergs in darkness or in fog. Even in 1959,
icebergs caused the sinking of an ocean liner fully
equipped with modem aids to navigation. Sir Hiram
Maxim, a prolific inventor who in the late nineteenth
century attempted to build flying machines, proposed
that bats' methods of navigation be copied directly in
the design of safety devices for ocean-going ships. Un-
fortunately, however, he did not really know how bats
navigate— for the simple reason that the subject had been
largely neglected since the days of Spallanzani. He sur-
mised correctly that bats used echolocation but was in-
correct when he assumed that the probing sound came
from the beating of their wings. Hence he advised that
ships generate very /ow-frequency sounds of roughly 15
c.p.s. and that receiving devices for such frequencies be
mounted on the bow of the ship. Faint echoes from this
sound were to ring a small bell, loud ones a large gong,
so that the crew could judge the seriousness of the
danger.

Maxim's idea was, nevertheless, a step forward in
understanding bat navigation, for it introduced for the
first time the idea that sounds inaudible to human ears
might be the basis of bats' uncanny abihty to fly in dark-
ness. But his ideas did not lead to any practical method
for detecting icebergs, and for at least two important
reasons. In the first place, the low frequencies which
he proposed meant that long wave lengths would have
been involved; 15-c.p.s. sound has a 20-meter wave. It

108

SONAR AND RADAR

is now well known that objects whose size is much less
than the wave length of the sound being used yield only
faint echoes, but in 1912 this was not a generally appre-
ciated fact. Had scientists been less scornful of bats and
had they known more about "Spallanzani's bat prob-
lem," more progress would have been made by 1912.
Furthermore, Maxim proposed to echolocate icebergs
through the air, whereas both the actual danger to the
ship and the major part of the iceberg lay beneath the
surface. The latter consideration led other inventors to
investigate the possibility of using underwater sound.

Two or three years after the sinking of the Titanic,
the increasing use of submarines by the German Navy
spurred the development of underwater sound devices.
At first it was largely a matter of Ustening to the sounds
originating from the submarine, particularly from its en-
gines and propellers. Much of the naval use of under-
water sound is still passive listening for the sounds of
other ships. But to a small degree by 1918, and to a
much greater extent by 1940, research had led to active
probing of the sea with sounds which would yield usable
echoes. Enemy submarines were the main military tar-
gets, but along with the development of sonar came the
echo sounder, or fathometer, a device to measure the
depth of the water. In comparison with an enemy sub-
marine (or the fish detected by porpoises), the bottom
of the ocean would seem to be an easy target, but for
many years the idea proved simpler than its realization.
In the deeper parts of the ocean even an echo from the
bottom was faint and diflBcult to detect with the early
sonar devices, but the most critical problem came when
the water was shallower and more dangerous. Here the
diflSculty was that the ship's hull had a disconcerting
tendency to "ring" or prolong the outgoing sound even
after the actual generating mechanism had been turned

109

ECHOES OF BATS AND MEN

off. The combined sound lasted much longer than the
time required for it to make the round trip to the bot-
tom. In other words, there were severe problems of dis-
cnw/wa//(7«— separating relatively faint echoes from the
continuing, original emitted sound. The instruments
were confronted with the same problems as those that
make a blind man less skilled at echolocation than a
porpoise or a bat. This engineering problem was solved
in part by learning how to make underwater sounds of
shorter duration.

By the 1950s, however, echo sounders had been per-
fected to a level of reUability where they have become al-
most essential for safe navigation. They even became so
sensitive that they began to indicate "false bottoms" be-
tween the ship and the true bottom. "Finding" two or
three extra ocean bottoms above the real one was a
rather disconcerting discovery, but after a time the fish-
ermen who used echo sounders began to notice that some '
of the "false bottoms" were really echoes from schools
of fish. Still later, mysterious layers of faint echoing, or
sound scattering, were noted almost everywhere in deep
oceans at several hundred feet below the surface. These
have been called the deep scattering layers and they were
later found to migrate up and down with dawn and dusk.
This fact provided the clue to their identity.

Oceanographers had already discovered by systematic
netting operations that large populations of shrimp and
other small marine animals five at depths where sunlight
barely penetrates. This depth is greater at noon than at
midnight; hence there is a massive vertical migration of
these animals upward during the evening and down
again at daybreak. The physical records of the deep
scattering layers turned out to match the known be-
havior of the animals. Once this additional fact was es-
tablished, the echo sounder became a valuable tool for

110

SONAR AND RADAR

biological research, for now the timing of the vertical
migrations could be studied with great precision. Of
course the echoes from a deep scattering layer do not
identify the actual animals, so we still do not know for
certain whether the principal sources of these echoes are
shrimp-like animals, fish, or possibly squid.

Sonar systems effective at echolocating submarines
were used with great success in World War II. One of
these sonar systems has a transmitting hydrophone, or
underwater loudspeaker which broadcasts sound whose
power level is 600 watts. For comparison, the minimum
sound power level audible in a quiet room at the fre-
quencies to which the human ear is most sensitive is
10-16 ^att, while a very loud shout at close range has
a power of 10~^ watt. Thus this sonar system puts into
the ocean a sound power roughly equivalent to that of
6,000,000 loud shouts. These intense probing sounds
are emitted as short pulses lasting one or two tenths of
a second. The frequency can be set anywhere between
10,000 and 26,000 c.p.s. Since the velocity of sound in
sea water is about 1500 meters/second, the actual wave
lengths of these sounds are from 5 to 13 centimeters,
and the length of the pulse is from 150 to 300 meters.

Because this system emits some frequencies above
the range of human hearing, there has to be a system to
make these frequencies audible. You may be famihar
with the "beat note," or "beat frequency" which is con-
spicuous when two nearly identical notes are sounded
simultaneously. If one note is 500 c.p.s. and the other
is 600 c.p.s., you will hear a third note of 100 c.p.s.
Hence in the electrical circuit of the sonar apparatus,
by generating a local frequency and combining with it
the incoming echo, an audible beat note is generated.
For instance, an incoming echo of 22,000 c.p.s. and a
local frequency of 23,000 c.p.s. produce an audible note

111

ECHOES OF BATS AND MEN

of 1000 c.p.s. Since the emitted sounds were of short
duration, the beat note was also short and sounded Hke
"ping." So familiar was this noise to antisubmarine
sailors that probing with sound came to be called
"pinging."

In selecting the frequencies of the underwater sound
which will produce the most useful echoes, the same gen-
eral considerations apply as apply to echolocation by
bats or bUnd men. Short pulses are desirable because
they allow the emitted sound to end before the echo re-
turns. This means that the frequency of the waves within
the pulse cannot be too low; otherwise the pulse duration
allows time for only one or two sound waves. Even sub-
marines are small enough targets that long wave lengths
could become inefficient owing to the smaller echo re-
turned by an object smaller than one wave length.
Furthermore, the background noise always present in
the sea is greater at lower frequencies. On the other
hand, in water as in air, there is an increasing loss of
sound energy as the frequency increases because of the
absorption of sound as it travels through the water. Bats
have evolved a most satisfactory machinery for echolo-
cation, but men designing sonar systems had to balance
all these factors against one another in reaching the com-
promise choice of 10,000-26,000 c.p.s. as a useful range
for practical echo ranging.

In view of the fact that many of the most successful
bats use signals with a rapid frequency change during
each brief pulse of sound, it is interesting to find that
sonar engineers developed a somewhat similar procedure
which sometimes improves the performance of the sys-
tem. In one type of operation the frequency of the
emitted sonar signal was varied continuously from 800
c.p.s. above to 800 cycles below the regular frequency.
This change was made to occur, as it does in the pulses

112

SONAR AND RADAR

of the FM bats, during each individual pulse of sound.
When the echoes of such pulses were received, the
frequency change was audible in the beat note. In
one typical setting of the apparatus the transmitted fre-
quency, and of course the echo, was varied from 20,800
to 19,200 c.p.s. If the local frequency was set at 19,000
c.p.s., the beat note would vary from 1800 to 200
c.p.s. and this would produce an extreme chirp or
"Wheeoough" sound. One advantage of this type of op-
eration was that at any particular instant of time the
many reverberations or multiple echoes from the ship's
hull and the water surface had traveled different dis-
tances and hence had different frequencies as they ar-
rived at the receiving hydrophone. This tended to create
an audible difference between the important chirping
echoes from a submarine and the noise level of rever-
beration from which they had to be discriminated. The
important echo was a clear chirp, the competing rever-
berations an irregular and shifting mixture of frequen-
cies. Very likely bats obtain a similar advantage from
their frequency-modulated pulses.

In another type of operation the sonar system used a
constant frequency in the emitted pulse, and the opera-
tor listened for slight differences in the pitch of the
audible beat note. Slight differences between the echo
frequency and the local frequency can produce large
changes in the audible ping. These differences can be
used to determine the relative motion of the target by
means of what is called the Doppler effect. This change
in frequency resulting from the motion of the source
causes the rising pitch of a train whistle as the train ap-
proaches you. To understand the Doppler effect, let us
consider a concrete example. Suppose that the sonar
ship is moving east at 10 meters per second while emit-
ting a 0.1 second pulse of 20,000 c.p.s. sound, that is,

113

ECHOES OF BATS AND MEN

2000 sound waves altogether. Let us simplify our arith-
metic by assuming that the velocity of sound in sea water
is exactly 1500 meters per second. If the ship were
stationary, the pulse would occupy 1500 X 0.1, or 150
meters of distance through the water. But it is moving
at 10 meters per second, or 1 meter in the one tenth
second required to emit the 2000-wave pulse. Since the
transmitting hydrophone pursued the sound waves and
covered 1 meter while emitting the pulse, the train of
waves was thereby compressed into 149 meters instead
of 150. This does not affect the velocity of sound in sea
water, so that a passing porpoise would hear the pulse
as 2000 waves occupying 149 meters and traveling like
all other sound waves at 1500 meters per second. All
the waves of the pulse strike the porpoise in 140/1500,
or 0.099 second, and their frequency would therefore
be 2000 waves in 0.099 second, or 20,202 c.p.s. In
other words, the emitted pulse has a higher frequency
to the listening porpoise because the ship has moved
during the process of emitting it. The velocity of sound
depends entirely upon the medium in which it is travel-
ing, not upon the velocity of the sound source.

Let us carry our example a little further and suppose
that this pulse strikes a submarine which is moving
west, toward the sonar ship, also at 10 meters per sec-
ond. The pulse, which was 149 meters long as it passed
the porpoise, is further compressed during the 0.1 sec-
ond while it is coUiding with the oncoming submarine.
As it is bouncing back from the target, it is again com-
pressed, both times by the same factor of 149/150. It
is not necessarily easy to see why this compression oc-
curs twice on striking the submarine, but an imaginary
modification of the physical events may help. Suppose
that the submarine did not return the echo by immediate
reflection but rather was equipped with a hydrophone

114

SONAR AND RADAR

and tape recorder so that the pulse was stored on tape.
Suppose that at some later time this recording was
played back into the water. The compression would oc-
cur during both reception and rebroadcast of the sound
waves, since in both cases the submarine would be mov-
ing relative to the water. Now suppose that the delay
between recording and playback is made less and less.
Nothing we do while shortening the delay time would
affect the compression of the train of sound waves, so
that there will still be two such compressions regardless
of whether the delay is long or short. If the delay is very
short, it approaches zero, and zero delay brings us back
to the original situation of immediate reflection. Thus
the porpoise hears the echo as 2000 waves occupying
only about 147 meters. To be sure, one can spUt hairs
and say that 150 X 149/150 X 149/150 X 149/150 are
a very little more than 147. But it is not much more, and
I promised to keep our arithmetic as simple as possible.
Finally the 2000 sound waves reach the receiving
hydrophone of the sonar ship, which is still advancing
at 10 meters per second to meet them, and the same
compression is repeated for the last time. The end re-
sult is that the receiving circuit of the sonar system gets
the 2000 waves in a shorter time than was required to
send them out. The amount of this shortening is

149
0.1-0.1 ("jTfT^*' °^ approximately 0.03 second.

The Doppler effect can be somewhat simplified by
considering only the relative motion of the sonar system
and its target; in this example the two were approaching
at 20 meters per second. The pulse length of the received
echo is then reduced by the square of the ratio of the
relative velocity of approach to the velocity of sound.
It is obvious that if the two ships were moving away
from each other, the Doppler effect would work in the

115

ECHOES OF BATS AND MEN

opposite direction, and the net effect would be a reduc-
tion in the frequency of the echo.

To return to our specific example, the final echo has

150
a frequency at the sonar ship of 20,000 X (tt^)*? ot

about 20,540 c.p.s. If this is translated into an audible
ping by combining it with a local frequency of 19,000
c.p.s., the echo beat note will be 1540 c.p.s., whereas
if both ships were stationary, the beat note would be
1000 c.p.s. This is a fairly extreme example of rapid
approach of the two ships, but in actual practice sonar
operators can tell when a submarine turns or even when
it speeds up or slows down. Though we understand far
less of what goes on in a bat or porpoise brain than we
know about the operation of this sonar system, it is rea-
sonable to infer that similar comparisons of outgoing
and echo frequencies may well be used to detect the
motion of flying insects or swimming fish. The horse-
shoe bats with their constant frequency pulses can per-
haps make better use of the Doppler effect than can the
FM bats, but even the latter seem to use less frequency
sweep when closing on insect prey than during cruising
flights when they are presumably seeking to make their
initial contact and detection.

Prospecting by Echo

Sound waves are not limited to air and water; they
can also travel through solid materials of any kind. Even
the echo sounder designed only to echolocate the bot-
tom may sometimes show a type of false bottom differ-
ent from the fish echoes or deep scattering layer de-
scribed earlier. Sometimes the records indicate a second
or third bottom below the real one rather than above it.
This means that after the bottom echo of the probing

116

SONAR AND RADAR

pulse has returned to the ship's hull a further echo re-
turns somewhat later. On first seeing such a record, an
experienced physicist might surmise that the pulse had
made two round trips through the depth of water under
the ship's hull— down to the bottom, up to the surface,
down to the bottom again, and finally back as a second
echo. This can indeed happen, but then the time of ar-
rival of the second echo is almost exactly twice that re-
quired by the first. Many of the false bottoms that seem
to lie below the real bottom result from echoes return-
ing at other times than twice the travel time of the first,
direct echo. What really happens under certain condi-
tions is that some of the sound energy penetrates into
the mud or sand of the ocean floor, travels downward
through it, and is then reflected back again by some
sudden discontinuity such as a layer of rock of different
hardness or density. Making due allowances for the
velocity of sound transmission through the material
just below the bottom of the ocean, geologists can esti-
mate rather accurately the depth below the bottom at
which this discontinuity occurs. Without even intending
to do so, designers and users of echo sounders have thus
hit upon a method of echolocation underground.

Quite purposefully and for many years, other geolo-
gists have been studying the transmission of sound waves
through miles of the earth's crust. Earthquakes produce
vibrations that can be detected by deUcate vibration de-
tectors known as seismographs. So do man-made ex-
plosions if they are sufficiently violent. Blasting in mines
and quarries can be detected miles away, and the
seismographic detection of nuclear explosions has now
become a matter of major importance, a hotly debated
issue at international conferences. By comparing the
vibration records resulting from earthquakes at different
points around the world, it is possible to deduce that

1 17

ECHOES OF BATS AND MEN

some waves travel close to the surface, others through
deeper layers of rock, while still others travel hundreds
of miles below the surface. Careful study of the times
of arrival of such waves at different Ustening stations
has enabled geologists to learn much more than they
could have determined by any other method about the
composition of our planet. (The Science Study book
How Old Is the Earth goes into this subject in more
detail.)

The actual waves recorded by a seismograph are of
quite low frequency, and they are usually so irregular
that it is difi&cult or even meaningless to describe them
in terms of frequencies. Major components vary from
about 0.5 to 5 c.p.s. They also differ from sound waves
in air or water in that they involve motion in directions
other than the direction of wave propagation. There are
several different types of seismic waves, classified accord-
ing to the relative degrees of motion in various direc-
tions. By painstaking analysis of recordings made at
various points above and below the ground and in dif-
ferent directions from the place of a test explosion,
geologists can locate many kinds of rock structures be-
low the surface. This procedure has been of great use in
prospecting for oil, or rather for the types of rock and
salt deposits that are commonly associated with it. Much
of our industrial economy has been made possible by
the success of this method for echolocating oil.

Echoes versus X-rays

Sound waves have also come into widespread use for
harmless testing of materials such as metals and rubber.
If the material is pure and homogeneous, it transmits
sound waves in a smooth and orderly way. But if there
are discontinuities, such as air bubbles in castings or

118

SONAR AND RADAR

defects in tire casings, they distort transmitted sound
waves. In some cases very short pulses of sound are used
to produce distinct echoes in the material being tested.
The sound frequencies are often very high, up to 1
megacycle per second (10^ c.p.s.)? and this is possible
because relatively short distances of transmission are in-
volved. It is a comparatively inexpensive method of test-
ing compared to structural failure of an important and
costly machine, and the material is not damaged in any
way.

Recently this sort of acoustic probing has been ap-
plied to the living bodies of animals and men. It is pos-
sible to detect discontinuities in our internal organs in
this way, using sound waves generated at the surface of
the body by suitable sound sources, such as crystals
which are vibrated at high frequencies by electric cur-
rents. This method is not without its dangers, for intense
sound waves in our bodies can produce damage. But,
when properly controlled, the method has some advan-
tages over X-rays. At least any damage is local and, inso-
far as we know, is not a long-delayed effect on our
genes— the complex molecules in our reproductive or-
gans, some of which may in time determine what our
children will be like. One hmitation of this method stems
from the large number of discontinuities that are natu-
rally present in a human body— those between muscle
and bone, digestive tract and blood vessels, heart and
lungs, etc. Thus any abnormaUties must be discriminated
from a complex background of natural structures, and
this makes it more difficult to locate a tumor in a human
brain than an air bubble in a cast-iron pipe. Neverthe-
less, this new means for studying our invisible insides
may lead in time to safer or more effective methods of
locating internal disorders in an early and curable stage.
The discrimination problems may be no more difficult

119

ECHOES OF BATS AND MEN

than those facing a bhnd man or a bat, and human m-
genuity may eventually solve this type of problem along
with the others mentioned in previous chapters.

Radar

The detection of distant aircraft by echoes of radio
waves stands as one of mankind's major technical ac-
complishments. In miUtary results alone it has well re-
paid the billions of dollars spent on its development
and on manufacture of military radar systems. Not only
can ground- or ship-based radar systems detect airplanes
at hundreds of miles but smaller radars carried on air-
planes can locate other aircraft and also resolve a sur-
prising amount of detail on the ground below. Radar
systems developed for the purpose can draw crude but
highly useful maps of hundreds of square miles of ter-
rain in a fraction of a second. The maps are drawn
on specialized cathode-ray oscilloscope screens. Radar
echoes can also be used to locate and track clouds and
storms, birds and locusts, meteors, earth satelUtes, and
ballistic missiles. Shortly after World War II, radar
echoes were successfully detected from the moon. In
1958, for the first time, very faint echoes from the
planet Venus were detected. Although this book can-
not discuss radar thoroughly, certain basic similarities
are well worth considering, and it is even possible to
make a rough comparison of the performance and effi-
ciencies of radar systems and natural hving systems that
have evolved to enable bats to navigate and catch insects
in the dark.

Relative Efficiency of Bats and Radar

As with the sonar system we discussed, this compari-
son wiU be based on radar systems that served well in

120

SONAR AND RADAR

World War II and have since been retired to pasture-
replaced by somewhat more efficient models. To make
the comparison more meaningful, I have selected a
typical airborne radar set which was a real triumph of
engineering skill in that it accompUshed, with a relatively
small weight and power consumption, as much as many
previous models that were far bulkier and less efficient.
This radar operated at a frequency of 9375 megacycles

(X-3x X IQQ), or a wave length of 3.2

y.j /J

centimeters. While this frequency is vastly higher than
those used by bats, porpoises, or naval sonar systems,
the wave length is not greatly different because of the
much higher velocity of light or other electromagnetic
radiation. Where our sonar system emitted its acoustic
signals at a peak power level of 600 watts, this radar
developed a peak power of 10,000 watts. It is important
to stress that none of these systems, living or instrumen-
tal, emits power continuously; all have a relatively low
duty cycle, or ratio of time on to time off. In typical op-
eration this radar emitted pulses lasting 0.8 microsecond
(8 X 10~^ second) at a pulse repetition rate of 810 pulses
per second. In other words, every 1/8 10th second, or
1.23 X 10-3 second it emitted a pulse lasting 8 X 10""^
second, followed by a silent interval about 1500 times
as long. This left ample time for echoes to return (at
the velocity of light) before the next pulse arrived. The
entire radar system weighed 124 pounds, but this does
not include the weight of the airplane generator which
supplied the electric power. This radar set detected air-
craft at 50 miles under most conditions and was a bril-
liant operational success. It is therefore of some interest
to inquire how well it compares with bat systems, watt
for watt of power emitted and gram for gram of weight.
This comparison is not a simple one because of the

121

ECHOES OF BATS AND MEN

widely different circumstances in which the two classes
of echo-ranging systems are actually used. Bats are in-
terested in detecting small insects at a few feet or yards.
The user of an airborne radar wishes to locate objects on
the ground and other airplanes some miles away. Bats
use sound waves, while radar employs radio waves of
only sUghtly greater wave lengths. Bats maneuver very
rapidly, the whole sequence of detection, turning toward
an insect, intercepting, catching, and swallowing, all oc-
curring withm 1 second. In ordinary use of an airborne
radar, the operator sees a spot on his oscilloscope
screen, notes how it changes in position relative to his
own flight path, and then takes appropriate action. This
may vary all the way from a turn to avoid any danger of
collision, if the two airplanes are airhners, to a close
pursuit and firing of a machine gun or rocket at the
other plane if it is an enemy in time of war. In both
cases the whole operation may be accompUshed by a
man sitting in a darkened cabin looking only at spots
on his radar screen. The bat does it all within one sec-
ond, in the dark, with a brain smaller than the eraser
on a pencil.

To make comparison a quantitative one, we can best
tabulate the important quantities which are known for
the two systems and on which we may base estimates
of their relative efficiencies. The table on page 123 gives
approximately the range of the radar and also its weight
and power requirements. An efficient system for echo-
location should detect the smallest possible objects at
the greatest possible distances and it should do so with
the least possible power and the lightest possible appa-
ratus. Bulky installations of whirling machinery may be
impressive at first glance, but unnecessary complexity
and power expenditure are actually signs of inefficiency.
With this in mind, let us set up an efficiency index, an

122

SONAR AND RADAR

equation which will evaluate the combination of these
four important factors. Such an index should have a
high value for the most efficient systems and should be
roughly proportional to the relative efficiencies of the
various systems of echolocation that we compare. As
will become clear a Uttle later, this is not as simple as
it might seem, but the process of attempting to define
such an index, and then modifying it as may seem nec-
essary, will in itself prove to be helpful in calling atten-
tion to the various quantitative considerations that are
important for echolocation.

TABULAR COMPARISONS OF BATS AND RADAR

AN/APS - 10

radar
system

Big

brown
bat

Target detected
Target diameter, d

(cm)
Range of detection, R

(cm)
Weight of apparatus, W

(grams)
Emitted power, P

(watts)
R/PWd
RVPWd^
RVPWd^

Airplane Insect

300

8X10^

9X10*

10*

2.9 X 10-^

5 X 10^3

5.5 X 108

200
0.1

Little

brown

bat

Wire

1.8 X 10-2

0.05

10-5 10-6

2 X 108 iQii

1.6X10^5 3.8X10^^
1.6X1015 1.2X1022

The above table Usts the range of detection, R,
the diameter of the target, d, (both in centimeters), the
power emission, P (watts) , and the weight of the system,
W (grams). For the bats, 10 per cent of the weight of a
fasting animal seems a generous allowance for the lar-

123

ECHOES OF BATS AND MEN

ynx, ears, auditory portions of the brain, and the other
parts used directly for echolocation. For both bat and
radar the power is the peak level reached during each
pulse. It may be recalled from Chapter 2 that the ears of
bats and men operate at sound power levels ranging from
about 10"^^ to 10"* watt per square centimeter. The air-
borne radar detection of another airplane at 50 miles is
compared with two cases of echolocation by bats— the
detection of a 1 -centimeter insect (or pebble) by a big
brown bat at 2 meters, and the echolocation of a 0.18-
millimeter wire by a Uttle brown bat at 90 centimeters.
The j5rst approach to defining the efficiency index
might be simply to have R, the distance of detection,
in the numerator, and the other three quantities, P
(power), W (weight), and d (target size), in the de-
nominator, where large values will tend to lower the in-
dex. This index, R/PWd, is listed in the next row of the
table, and when judged on this basis, the bats appear bil-
lions of times better than the radar system. But a little
reflection shows that, in defining the index in this way,
we have made an important assumption; namely, that
these four quantities are really related to one another as
we have entered them in the equation. For example,
this definition of efficiency assumes that range will in-
crease in direct proportion to power. But for all radar
systems, and probably all bats, the emitted energy falls
off as the square of the distance. And most small targets
send back echoes that also obey the inverse-square law.
As pointed out in Chapter 4, where insect-catching bats
were compared with the hypothetical case of a bat at-
tempting to echolocate fish through the air-water inter-
face, the energy in an echo is proportional to 1/R*.
This means that to obtain twice the range a system of
echolocation will need 2*, or 16 times as much power,
and we should therefore change our index to contain R

124

SONAR AND RADAR

to the fourth power instead of the first. This will greatly
increase the rating scored by the radar set detecting an
airplane at 50 miles.

Having made this improvement in the index, we
should also scrutinize the other variables in our equa-
tion, in particular the size of the target, d. If a series of
targets is fairly large relative to the wave length of the
signal being used to generate an echo, the echo power is
usually proportional roughly to their areas, or to d^.
This is true of most radar targets, and certainly of air-
planes being echolocated with 3.2-centimeter waves. Is
it also true for bats? The insects they catch vary from
somewhat below one wave length to several wave
lengths, and of course the FM bats employ orientation
sounds containing a whole octave of frequencies, or a
twofold range of wave lengths in each pulse. It is prob-
ably reasonable to assume that in insect detection the
echo power varies as the square of the target diameter,
although in some cases the insects may be enough below
one wave length so that this assumption would lead to
an overestimate of the echo strength. The next line of the
table therefore lists for each of the three systems the
value of the revised efi&ciency index, R^/PWd^. Even
on this basis the bats are somewhat superior to the radar.

Finally, we should pay a little more attention to the
bats which detect wires far smaller than one wave length,
such as the little brown bat listed in the third row of
the table. When wires or other cylindrical obstacles are
much smaller than one wave length, the echo power var-
ies as d^, and the 0.1 8 -millimeter wires detected at 90
centimeters are certainly in this size range. This domain
of target size produces what is sometimes called Rayleigh
scattering, after the nineteenth-century physicist who
analyzed it with special reference to Ught scattered by
tiny particles in the air. Such light makes up most of

125

ECHOES OF BATS AND MEN

what we see in the sky, and since the particles are of less
than the wave lengths of visible light (4 to 7 X 10"^
centimeter), short wave length light is more strongly
scattered than other colors. This is why the sky is blue.
By analogy we might say that the bat flying up to these
wires must hear "blue echoes." In any event, a case could
be made for evaluating bat sonar by means of an index
containing d* rather than d or d-, and the value of
R^/PWd^ is therefore listed in the last Une of the table.
The drastic results of changing the definition of our
efficiency index should now be clear. This may indeed
open serious questions as to whether such different sys-
tems for echolocation can be meaningfully compared on
a simple numerical basis. Furthermore, several other im-
portant factors have not yet been brought into the com-
parison. Bats operating with sound waves in air face
serious reductions in echo signal due to the absorption
of sound in air, especially at higher frequencies. During
the round trip from bat to target and return, sound of
50 kc loses power by a factor of 0.63 for every meter
of distance, in addition to the reduction due to the in-
verse fourth power reduction for echoes. At 100 kc the
reduction is by a factor of 0.44 over every meter. Radio
waves suffer no such severe losses in traveling through
the air. This fact puts the bat at a great disadvantage
over long distances. On the other hand, there is a con-
sideration which would favor most radar systems as
compared to bats. This is the duty cycle, or fraction of
the time during which energy is being emitted. In typical
cases, such as those included in our table, a bat would
be emittmg 10 to 20 pulses per second, each pulse lasting
2 to 5 milliseconds, so that the duty cycle would vary
between 0.02 and 0.1. The radar had a far lower duty
cycle, however, the interval between pulses having been
about 1500 times as long as the pulse itself, so that the

126

SONAR AND RADAR

duty cycle would be about 0.0007. This means that if
we were to use average power rather than peak power
in our comparison, the bats would suffer by a factor
of about 100. Yet a partisan of the bats might offer in
rebuttal the consideration that we allowed 10 per cent of
the animal's weight for its sonar apparatus, whereas the
weight of the radar set was a much smaller fraction of
the mass of the airplane that carried it. From the bat's
point of view it would perhaps be more valid to compare
its whole weight with that of the entire airplane.

If we take the broadest view, it is obvious that bats
and other Uving animals are vastly more efficient than
radars and airplanes, even though it is difficult to attach
numbers to the comparison. Bats maintain and repair
their Uving machinery; airplanes and radar sets must be
manufactured and repaired by men. Bats catch and di-
gest all the food that provides power for their bodily
mechanisms; airplanes are not expected to refuel by
catching birds, and the fuel pumped to them requires no
chemical processing in the plane before use. Nor do any
artifical mechanisms reproduce themselves. The unusual
aspect of the comparison we have been making is that a
Hving mechanism can be compared directly with a radar
set on almost the same terms that an engineer would
employ in comparing one radar with another. The re-
sults of the comparison inspire a healthy respect for the
mechanisms of flesh and blood which have evolved in
nature under the pressure of natural selection.

127

CHAPTER 6
Suppose You Were Blind

In the preceding chapters we have examined waves and
echoes to understand better how animals and men have
used them to locate objects which are essential for sur-
vival. Such studies of natural phenomena often seem use-
less to all but a very few people, but so do many scientific
explorations. Yet history has clearly shown that men
have improved their lot by investigations into the un-
known. However insignificant it may have seemed at the
time, there is a true inner satisfaction in discovering new
relationships and new information to add to our under-
standing of the world around us. We often hope that
observations and new facts can one day be used to im-
prove our environment still further. What could be more
beneficial than trying to apply this new-found knowledge
to men who cannot see with their eyes? Can we help
them to "see" with their ears— to learn the language of
echoes?

Blindness is always a tragedy for human beings be-
cause our brains and our whole way of life are built
around light and vision. But men's eyes are not their only
channels of communication with the rest of the world,

129

ECHOES OF BATS AND MEN

and sound is in some ways even more useful. For exam-
ple, we can see somewhat less than 1 octave of frequen-
cies, or wave lengths, roughly from 4 to 7.5 X 10""^ centi-
meter. Our sense of hearing, on the other hand, extends
from about 20 to 20,000 c.p.s., a range of a thousand-
fold, or approximately 10 octaves. Audible sound can
thus contain a much richer variety of frequencies than
visible light, and this is partly why sound rather than
Ught is used for speech. Of course, there are other rea-
sons; for instance, living organisms cannot generate
hght, except for a few luminescent animals and plants.

The sharp shadows cast by light make it less useful as
a vehicle for speech and short-range communication.
Just because sound does go around comers, it is useful
in calling and signaling, particularly when almost every
motion and contact between a person or animal and the
physical world around it generates some sound. The
great advantage of light to us is that it has short wave
lengths and, consequently, objects of small size give off
specular reflections. It is for this reason that eyes and
lenses can focus sharp images. Only when one tries to
use a microscope to see objects about the size of the
wave length of light does that wave length become an
important Hmitation. An object must be smaller than
one micron (one millionth of a meter) before it scatters
light rather than reflecting it.

If sound waves and light waves did not already exist,
we well might find scientists trying to invent them, one
to form sharp images and permit accurate observations
of small details, the other with a wide-frequency spec-
trum to convey complex information with a minimum
of interference from shadow-casting obstacles. The two
types complement each other, and while the loss of our
sense organs for either is a major handicap, there is

130

SUPPOSE YOU WERE BLIND

enough duplication of what each can do to permit some
substitution of one after losing the other.

The Sense of Obstacles

Blindness has been an all-too-common aflSiction of
men, and while no device or procedure can completely
replace lost sight, blind men for centuries have learned
to get about in the world and carry on a surprising num-
ber of activities. Some become so skillful at avoiding ob-
stacles and maintaining an adequate general orientation
that it is difficult for a stranger to realize they are really
blind. For example, there was once a blind boy who
learned when six years old to ride his tricycle all about
the sidewalks near his home without injury or accident.
When he approached pedestrians, he steered around
them, and he always knew when to turn corners without
going into the street. Other blmd people travel widely
in busy cities, crossing streets, using buses and trains,
dodging lampposts and wire fences. How do they detect
these obstacles before touching them? Many theories
have been advanced, both by the blind people themselves
and by those who have worked or lived with them. Cu-
riously enough, the most skillful of the bUnd differ
widely in their explanations of their own abilities. Many
say they feel with then* hands or faces the proximity of
obstacles, and the term "facial vision" has come into
wide use to describe their orientation to objects which
are too far away to feel or touch. Others believe that
hearing is somehow involved; still others speak of "pres-
sures" and other ill-defined sensations that warn them of
dangers just ahead.

The central question is obviously the nature of the
physical message that travels from the obstacles to the
bhnd man, and the way in which his remaining sense

131

ECHOES OF BATS AND MEN

organs detect and interpret this information from the
outside world. From about 1890 to 1940 many studies
were made of the "sense of obstacles," but only in the
early 1940s was a conclusive answer obtained from care-
fully controlled experiments. While these experiments
were performed by men who called themselves psycholo-
gists, the experiments can be considered classic examples
of biophysics, the application to problems posed by liv-
ing organisms of the same basic principles of investiga-
tion that have developed physics as a rigorous science.
The chief difference between biophysics, thus broadly
defined, and the physics of non-living systems is the
greater degree of complexity and refinement of living
organisms. Animals and men are made of far more
intricate mechanisms than clickers and ripple tanks, mi-
croscopes or television sets, and this is why our under-
standing of biological processes is so much less thor-
ough and complete than our knowledge of physics or
chemistry.

The psychologists, or biophysicists, who finally solved
the question of obstacle perception by the blind were
Professor Karl M. Dallenbach of Cornell University, and
two graduate students, one of whom, Michael Supa, was
himself totally blind. Milton Cotzin, the other student,
had normal vision, but he and others who served as ex-
perimental subjects wore blindfolds for many hours at a
time in order to experience what life is Hke for the blind,
and, in particular, to develop as much as possible the
ability to detect obstacles before bumping into them.
First the experimenters set up a sort of obstacle course,
a long hallway down which the subject walked and across
which was placed a large screen of fiberboard at some
point chosen by the experimenter. This location was var-
ied from trial to trial, so that the subject never knew
whether it was 6, 10, 18, 24, or 30 feet ahead of the

132

SUPPOSE YOU WERE BLIND

Starting point, or even whether it was there at all. His
task was to walk along the hallway, say when he first
thought he was approaching the screen, and then walk
up as close as he could without striking it.

Some of the subjects, both blind and blindfolded,
could judge accurately the presence or absence of the
screen at several feet and then move in untU their faces
were within a few inches before deciding that any further
approach would bring them into contact with it. The
phenomenon of obstacle detection was thus brought into
the laboratory in a manner which allowed it to be studied
repeatedly under reasonably constant conditions. This
step is often a crucial one in attacking scientific problems
of this sort. Elusive and unpredictable events are very
much more difl&cult to study than those which can be
repeated under known conditions. Only in the latter case
is it fairly easy to vary the factors that seem likely to be
important and then observe the results. EarUer studies
of obstacle detection by the bUnd had been plagued with
great variability in the performance of the subjects. That
mainly is why they had not led to clear and decisive
answers. Yet Supa, Cotzin, and Dallenbach built their
experimental design on the extensive, if inconclusive,
experience of earUer experimenters. Without this back-
ground they would probably not have been able to devise
such decisive experiments.

Once they had arranged conditions where blind or
bUndfolded people were regularly detecting a standard-
ized test obstacle, the next step was the theoretically ob-
vious but nevertheless rather difficult one of eUminating
one possible channel of sensory communication at a
time, while leaving the subject with free use of the others.
One leading theory was that the skin supplied some kind
of sensation of touch or pressure when obstacles were
nearby; another was that sound played a major role. The

133

ECHOES OF BATS AND MEN

practical problem in testing the "skin pressure theory"
was to shield the subjects' skin from any possible in-
fluence that might be arriving from the obstacle, and
this was doubly difi&cult because no one could say what
this might be— air currents, electromagnetic radiation,
heat or cold, or possibly some sort of energy not known
to physics. To test the sound theory, the logical proce-
dure was to prevent sounds from reaching the subjects'
ears without interfering with whatever the skin might be
feeling as a result of proximity to the obstacles. The
covering of the skin clearly had to be accomplished with-
out interfering with the subjects' hearing, and vice versa.
The final outfit that the subjects were obliged to wear
consisted of a long veil of thick felt which covered the
head and shoulders, plus heavy leather gloves to shield
the hands. Ordinary clothing covered the rest of the body
surface. Such was the protection that they could not feel
even the air current of an electric fan directed at their
heads. After some preliminary trials to accustom them
to walking about in this "armor," the subjects found
they could detect the screen almost as well as ever. The
average distance of first detection had been 6.9 feet with
no veil or gloves, and it was now reduced only slightly
—to 5.25 feet. This seemed to dispose of the possibility
that obstacles were detected by feehng them through the
skin, despite the fact that originally some of this group
of subjects, like many blind people, were certain that
they felt the obstacles with the hands or face.

The next experiment was to leave the hands and face
completely free but to cover the subjects' ears. Earlier
experiments of this kind had given conflicting results;
sometimes the detection of obstacles was impaired,
sometimes not. Complete exclusion of sound by ear-
plugs is not possible, but Supa, Cotzin, and Dallenbach
wished to be sure that as httle sound as possible reached

134

SUPPOSE YOU WERE BLIND

their subjects. They therefore wore earplugs of wax and
cotton, ear muffs, and padding over the sides of the head.
This compound series of barriers was necessary because
many sounds, particularly those of low frequency, pene-
trate ordinary earplugs or ear muffs. Everyone knows
from the ordinary experience of wearing ear muffs or
parka hoods in cold weather that by speaking slightly
louder than usual one can still converse with his com-
panions no matter how well the ears are protected from
the winter winds.

So thorough was this muffling that the subjects could
not hear the sounds of their own footsteps, and instruc-
tions could only be given them by loud shouts. A loud
shout can easily have 10^ times the energy of a barely
audible whisper. Direct measurement of the intensity
necessary for them to detect a test sound showed that
their auditory sensitivity had been reduced by a factor of
about 4,000,000; that is, they could hear the test sound
only after its energy level had been increased four
millionfold above the level that was just audible without
the ear covering.

When the same subjects were now asked to repeat the
experiments with their hearing thus impaired, the results
were spectacular. None retained any obstacle -detection
ability at all, and in each of one hundred trials every
subject bumped unexpectedly into the screen. One of the
blind men, who had stoutly maintained that sound
played no part at all in his "facial vision," complained
that he was now getting no sensation at all, and for the
first time he walked hesitantly and held out his hands
to guard against anticipated accidents. If sound does ac-
count for the obstacle-detection ability, one might ask
why there was any reduction in distance of first detection
when the subjects wore the felt veil and leather gloves.

135

ECHOES OF BATS AND MEN

This was probably due to the reduction in sound level
caused by the shielding effect of the bulky hood.

Guiding Echoes

These experiments would seem to have settled the
matter once and for all, but criticisms would still have
been possible if the experimenters had stopped at this
point. Perhaps the pressure of the ear covering was dis-
turbing some subtle tactile sense. Perhaps blind men
were warned of obstacles not by hearing as such but by
some special kind of pressure sense involving the ear
canal or adjacent areas of skin. Even men who had stud-
ied this subject for years were skeptical that sound waves
could be the messengers by which blind people detected
obstacles. Further, many blind men themselves still con-
tinued to think they felt obstacles. To convince such
skeptics it was necessary to modify the experiment so
that sound and only sound carried the necessary infor-
mation from the outside world into the subject's nervous
system. This might seem a hopeless task; if the experi-
ments described above were unconvincing, what argu-
ments could hope to overcome such skepticism?

The answer was to employ a telephone system to
transmit the appropriate sounds to the subject sitting in
a remote and soundproof room. The sounds transmitted
over the telephone wires were those picked up by a mi-
crophone carried by a man walking along the same
obstacle course. They were similar, though not identical,
to what the man would hear himself if he were listening
for evidence that the screen was just ahead.

The results of the telephone experiment were aston-
ishingly close to those obtained by the same subjects in
the original tests. They could sit in the soundproof room
and decide by listening to the telephone whether the

136

SUPPOSE YOU WERE BLIND

screen was being approached or not. After some practice
they could detect the screen at an average distance of
6.4 feet, only a little less than their average of 6.9 feet
when they were doing their own walking and listening.
Such a result would seem to dispel all doubts; surely no
one could argue now that anything but sound was in-
volved. But scientists who have studied problems like
this have learned to be extremely cautious. Many experi-
ments which have seemed this convincing have turned
out to be misleading. Suppose, for example, that the per-
son who walked up to the screen with the microphone
changed his breathing rhythm or the sounds of his foot-
steps and thus unconsciously conveyed to the remote
hstener his proxunity to the screen? This sort of uncon-
scious signaling has been known to occur, and, inciden-
tally, it accounts for many cases of what has been
interpreted as mental telepathy.

This worry led to further experiments in which the
second person was replaced by a motor-driven cart
which carried the microphone towards the screen. The
subject in the soundproof room controlled the move-
ments of the cart while Ustening to the sounds the mi-
crophone picked up. As often happens in a scientific
experiment, new facts raise new questions— one often
ends up with more questions than he had at the begin-
ning. Here the question raised was of major importance.
Granted that sounds could be conveyed over the tele-
phone system, what were the actual sounds that told
the listener the screen was near? In the original experi-
ment no special effort was made to generate sounds or
produce echoes; indeed, the experimenters in the begin-
ning had been uncertain that sounds were really of any
consequence. They had simply tried to bring phenome-
non into the laboratory and arrange conditions under
which it could be repeatedly studied. But having learned

137

ECHOES OF BATS AND MEN

that sound, rather than anything which could not travel
along telephone wires, informed the blind man that the
screen was in front of him, the experimenters had to con-
sider the nature of these sounds.

Footsteps were an obvious possibility, and when the
original experiments were repeated with the subjects
walking in their stocking feet on a soft carpet, their
ability to detect the screen was greatly reduced. The av-
erage distance of first detection fell from 6.9 feet when
the subjects were wearing shoes and walking on the bare
floor to 3.4 feet when the sounds of their footsteps were
dampened by the carpet. Some subjects snapped their
fingers or made clucking sounds, but others apparently
relied on whatever sounds were present in the hallway,
such as the sound of their own breathing or the rustle of
their clothing. This question had not been seriously con-
sidered in the design of the first experiments, but now
that the investigation had reached the point where the
microphone was to be mounted on a cart there would be
no sound from footsteps or breathing. Some other sound
had to be substituted, which provided the opportunity to
study the usefulness of various sounds in providing audi-
ble clues to the presence or absence of obstacles. Ob-
viously, too, the experiment involved echoes. If some
sound told a listener that the screen was present, it must
have been a sound which was different with the screen
than without it.

In order to study the character of the echoes used by
blind people, the experimenters then equipped the cart
with a loudspeaker as well as a microphone. A variety of
sounds with known characteristics could now be gen-
erated by the loudspeaker for further tests. If a loud
hissing noise was used— that is, a noise containing a wide
range of audible frequencies— the screen could be de-
tected by the subjects hstening to the telephone in the

138

SUPPOSE YOU WERE BLIND

soundproof room. The distance of first detection aver-
aged 3.75 feet, less than the range of detection when in
an earlier test a person carried the microphone toward
the screen. Nevertheless, it was an impressive perform-
ance, considering how greatly the situation had been al-
tered from the first series of experiments. Other sounds
were also tried, but the experim.ents were concluded be-
fore the ideal sound had been discovered which men
might use to obtain the more revealing echoes. The in-
vestigations ended because the original problem had
been conclusively solved by the proof that sounds and,
in particular, echoes were the messages that inform blind
men about the existence and position of obstacles.

One significant feature of this important discovery is
the striking divergence between the subjective feelings of
many blind people and all the objective evidence which
we have examined. When a man has developed the re-
markable abihty to find his way about through the bus-
tling traffic of a modern city in what to him is total
darkness, and when he does this so skillfully and un-
obtrusively that one can travel with him for hours and
never suspect that he is blind, then it is natural to assume
that he knows what he is doing and how he does it. But
often the expert blind man can say only that he some-
how "feels" his way and "knows" before he bumps into
the tree or fence post that it is there. If questioned more
closely, he may say he feels the proximity of the object
with his hands, his face, or his forehead. Yet when the
process of obstacle detection is studied under controlled
conditions, it is clear that sounds and hearing are the
essential ingredients. In addition, the whole surface of
the blind man's skin can be covered by heavy felt or
leather without preventing him. from detecting obstacles
before he strikes them. When his ears are plugged, he
no longer "feels" the obstacles with his hands or face,

139

ECHOES OF BATS AND MEN

and if he continues on his way, he invariably strikes them
without warning. Subjective impressions obviously can
be misleading-we do not always know just which of our
senses is informing us about our surroundmgs. This is
not to say that our senses are not keen, but rather that
our conscious thinking about them may lead us to the
wrong conclusion about how they operate.

This is not a unique misapprehension concerning the
workings of our sense organs, although perhaps it is an
extreme one. Another example also involves the sense
of hearing. How do we know where a sound is coming
from? Sometimes we see the source and are thus in-
formed of its position, but everyone is able to locate the
origin of an unfamiliar sound heard in darkness, and
usually with great accuracy. Sometimes we locate a
sound source approximately by turning our heads until
the sound is louder in one ear than in the other, but more
often and with great precision we rely on the difference
in the same sound as it arrives at the two ears. Consider
for the present only one type of sound, a sharp click.
The most important property of the bundle of sound
waves constituting the click is the time of arrival of the
first sound waves at the two ears. If the click comes from
straight ahead, the two ears receive the first sound waves
at exactly the same time because they are equidistant
from the source. If, however, the click arises at some
point to the right of the direction you are facmg, it
reaches the right ear a small fraction of a second sooner
than the left. If the source is 90" to one side, the opposite
ear is about 20 centimeters farther away than the closer
one, and since sound waves in air travel about 30 centi-
meters per millisecond, this means that the maximum
possible difference in time of arrival at the two ears is
less than 1 millisecond. Yet such is the precision of the
auditory portions of our brains that we can easily dis-

140

SUPPOSE YOU WERE BLIND

tinguish between a sound source that is straight ahead
and one that is displaced only 10° to one side. If the
source is 3 meters away, the 10° displacement which is
clearly noticeable involves a difference in time of arrival
at our two ears of about 0.1 millisecond. It is difficult

Fig. 15. Your ability to discriminate minute differ-
ences in the time of arrival of two sounds can be
tested with this device. Any source of sharp clicks will
do if it is tightly enclosed in the box so that you hear
it only through the tubes.

to locate sound sources accurately if they he directly in
front of us, or anywhere in the plane that is equidistant
from the two ears. If we have to attempt this, we usually
do so by moving our heads about and bringing one ear
closer to the source.

The role of differences in time of arrival of a click at

141

ECHOES OF BATS AND MEN

the two ears can be studied with the aid of a simple
device illustrated in Fig. 15. This consists of a source
of clicks, which could be a loudspeaker or a mechanical
clicker, and a sound-tight box to house it. From the box
lead two tubes each ending like a physician's stethoscope,
but make sure the earplugs are soft to avoid accidental
injury to your ears. One tube is fixed in length, while the
other has a telescoping tube like that of a trombone so
that its length can be varied. When the two tubes have
different lengths, it will obviously require longer for the
first sound waves of the click to reach one ear than the
other. Since the velocity of sound is known, the differ-
ence in time of arrival of the cUcks can be calculated
easily from the difference in length of the two tubes.
When one listens to clicks through these tubes of un-
equal length, the effect is strikingly Uke that of a cHck
coming from one side. If the eyes are closed and one
makes even a small effort to imagine that the clicks are
coming through the open air rather than through the
tubes, there is a compeUing illusion that the source is at
the side of the ear receiving the shorter tube. Of course
it makes no difference where the box containing the
source of cHcks is really situated, nor does the actual
length of the two tubes matter. When precise measure-
ments are made with more refined apparatus of this same
type, the minimum time difference that leads to this illu-
sion of a source at one side or the other is less than 0.1
millisecond.

We need experiments like these to tell us about one of
the principal ways in which we locate the source of a
sound. We never think, "That click reached my right ear
1/10,000 second before it got to my left ear; therefore,
it must have come from a little to one side of the median
plane of my head." We simply recognize that the click
came from one side without any idea how we located

142

SUPPOSE YOU WERE BLIND

it. In much the same way, a bUnd man learns to antici-
pate colHsions with obstacles under certain conditions,
usually without realizing at all that these conditions are
the presence of audible echoes. Recognizing the prox-
imity of an obstacle and knowing from experience the
pain of bumping into it, he comes to believe that he felt
its nearness with his hands or face. All this adds up
to a warning not to interpret the workings of our sense
organs and our brains too hastily; they may be operating
in other ways than we are first incHned to think. But we
should not go to the other extreme and conclude that
measuring instruments will always improve upon our un-
aided senses. As we have learned, sense organs and
brains of men, porpoises, bats, and beetles accomplish
extremely difficult feats of detection and discrimination.
To return to the blind man's problems of orientation,
echolocation is certainly the technique by which skillful
blind people find their way in the "dark." But with the
general question thus answered, we are immediately im-
pelled to ask what type of sound will provide a blind
m.an with the most informative echoes. In the final ex-
periments by Supa, Cotzin, and Dallenbach where the
cart carried the loudspeaker and microphone up to the
test obstacle, it turned out that a hissing noise was more
effective than pure tones. But the average distance of
detection was only 3.75 feet instead of 6.9. Does this
mean that footsteps are more efficient sounds for this
purpose, or does it mean that the cart with the loud-
speaker and microphone was less easily controlled by the
remote listener? From the experiments described in
Chapters 2 and 3 it is clear that some sounds generate
more useful echoes than others, and that a very short
click has the advantage that it ends before the first echo
begins to return. But footsteps on the floor are not es-
pecially sharp clicks, even when the walker's shoes have

143

ECHOES OF BATS AND MEN

hard soles. Some blind men prefer shoes with metal heel
plates, perhaps because of the sharper footsteps that re-
sult. If you have carried out some experiments with
clickers, like the one illustrated in Fig. 7, it must be
obvious that if you, a rank beginner, can detect trees,
an experienced blind man can do at least as well. Many
blind men have used clickers of one sort or another—
aside from metal heel plates, the canes used by many
blind men to lengthen the reach of their hands are used
as cUckers by tapping the ground or pavement. The re-
sulting cUcks provide a standard noise which gives a
useful echo.

But we can properly ask whether footsteps, cane taps,
or even toy cUckers mounted in horns are really the best
types of sound for a blind man's purposes. Do they gen-
erate the most informative possible echoes or are there
other types of sound that would be superior? The ques-
tion is simply asked, but the search for a convincing
answer has been difficult and frustrating. Various types
of cHckers and portable sound sources have been built
and tested. Some, particularly the directional clickers,
have been used extensively by a small number of blind
men, including their inventors. But the results have been
far from satisfactory, and many users find it too difficult
to hear consistent echoes or find the added facility at
orientation not worth the embarrassment caused by a
conspicuous audible sound that calls attention to their
handicap. Yet almost every object that a blind man needs
to detect does interact in some way with audible sound
waves. This being so, why can we not devise a probing
sound which will produce audible echoes that are recog-
nizably related to the objects a blind man needs to locate?

One difficulty has already been called to our attention
in the experiment where tape recordings of clicks or
other impulsive sounds were played backward on a tape

144

SUPPOSE YOU WERE BLIND

recorder. This experiment demonstrated the effectiveness
of our built-in suppressor mechanism which renders
echoes far less audible because our ears are temporarily
insensitive immediately after a loud outgoing sound. A
multitude of echoes are clearly audible on reversed play-
back when they precede a sharp chck or pistol shot, but
are quite unnoticed when they follow the louder sound
as they do in ordinary life. Is this the major reason why
blind men fail to learn as much from echoes as they
theoretically should? And if so, could not some device
be developed to overcome this difficulty? No one knows
the answers to these questions, and they are good ex-
amples of the truism that no branch of science is com-
plete or finished. Perhaps some reader may have the
ideas and the opportunity to make further advances to-
ward a real solution of the bUnd man's problems of
orientation. Just because some men have failed so far to
find such a solution, others should not be discouraged
from new attempts, especially when the potential gains
to human welfare are so great.

145

FURTHER READING

Barnes, H. : Oceanography and Marine Biology. Lon-
don: George Allen and Unwin, Ltd., 1959.

This up-to-date elementary survey of oceanography
includes a chapter on the sounds of marine animals
and the use of sound for exploration of the ocean
depths.

Boys, C, v.: Soap Bubbles and the Forces Which Mould
Them. New York: Science Study Series, Doubleday
Anchor Books, 1959.

A small readable classic of science which will give
you, among other things, a better understanding of
surface tension.

Bowen, E. G. (Editor) : A Textbook of Radar. Cam-
bridge, England: Cambridge University Press, 2nd Edi-
tion, 1954.

This textbook contains more general background ma-
terial, including such fascinating subjects as radar
echoes from the moon and the use of radar in
navigation.

Buddenbrock, W. von: The Senses. Ann Arbor, Michi-
gan: University of Michigan Press, 1958.

A readable and authoritative account of sense organs
of all sorts of animals, from the eyes of scallops to
inner ears of men.

147

FURTHER READING

Carthy, J. D.: Animal Navigation. London: George

Allen and Unwin, Ltd., 1956.

This popular and readable book describes the orienta-
tion and navigation of insects, fishes, birds, and whales
as well as those of bats and domestic animals.

Fletcher, H.: Speech and Hearing in Communication.
New York: Van Nostrand, 1953.

This thorough and somewhat technical book sum-
marizes the extensive researches carried out at the Bell
Telephone Laboratories and elsewhere on the physical
properties of speech, the mechanisms of human hear-
ing, and the nature of hearing losses and deafness.

GriflSn, D. R.: Listening in the Dark. New Haven, Con-
necticut: Yale University Press, 1958.

Many aspects of the natural sonar of bats, birds, and
porpoises are discussed more fully than in this short
monograph, including the many different types of bats
and their orientation sounds, their pursuit and capture
of flying insects, fish, and other food. There are
chapters on echolocation by bUnd men, and on the
acuity of echolocation achieved by bats, including
their ability to hear faint echoes despite the presence
of louder jamming noises.

GriJBBn, D. R.: "Bird Sonar." Scientific American Maga-
zine, March 1954.
"More about Bat 'Radar.' " Scientific American Mag-
azine, August 1958.
These articles contain excellent illustrations and sup-
plement the chapters of this monograph dealing with
the natural sonar of bats and birds.

Horton, J. W.: Fundamentals of Sonar. Annapolis,
Maryland: U. S. Naval Institute, 1957.

148

FURTHER READING

This rather technical book describes the basic prin-
ciples and operation of electronic sonar systems as
they are used on ships. It explains the Doppler effect
and other basic phenomena of echolocation with spe-
cial reference to underwater sound.

Hurley, P. M.: How Old is the Earth. New York:
Science Study Series, Doubleday Anchor Books, 1959.

This modem book on radioactivity as an energy
source in the earth and as means of measuring time
also includes a piece on seismic waves.

Pierce, J. R., and David, E. E., Jr.: Man's World of
Sound. New York: Doubleday and Company, 1958.
A semi-popular book describing at an elementary
level the physical properties of sound waves. It dis-
cusses such matters as interference, the propagation
of sound, standing waves, etc., in a way which any
student of secondary school physics should have litde
diflBculty in understanding.

Ridenour, L. N. (Editor) : Radar System Engineering,
New York: McGraw-Hill, 1st Edition, 1947.

This is a general description of the radar systems de-
veloped during World War II by the Radiation Labo-
ratory at M.I.T. While parts of it are quite technical,
many chapters can easily be understood by any seri-
ously interested reader of this monograph. Radar sets
and systems are described and illustrated in sufi&cient
detail to permit, for example, the sort of comparison
with biological systems that were discussed in Chap-
ter 5.

Rummell, J. A.: "Modern Sonar Systems." Electronics
(Engineering Edition), January 1958, pages 58-62.
A brief survey of the apparatus used in sonar systems.

149

FURTHER READING

Witcher, C. M., and Washington, L.: "Echo-Location
for the Blind." Electronics, December 1954, pages
136-137.
A brief but complete description of one of the more
successful sound-generating devices used by blind
people to find their way about. The late C. M. Witcher
was himself blind and he devoted his engineering tal-
ents to improving such devices for his own use and
for the benefit of other bUnd people.

Zahl, P. A. (Editor) : Blindness. Princeton, New Jersey:
Princeton University Press, 1950.

A collection of articles by different authors discussing
the most important problems faced by bUnd peo-
ple, education for the blind, vocational rehabilitation,
talking books, guide dogs, guidance devices, and the
remote possibility of direct stimulation of the optic
nerves or visual areas of the brain.

150

INDEX

Acoustic energy, 46; in water,
19

Acoustic orientation, 32.
See also Echolocation
and Orientation

Acoustic probing, 116-20

Acoustics: architectural, 50;
echoes, 57-73

Aerial interception, 18

Air, 19, 40, 44, 97, 116;
molecules of, 39; sound in,
25

Airplanes, 17, 120

Amazon River, 21

Amplitude, 36

Angle of incidence, 77

Angle of reflection, 77

Animal Navigation, 148

Animals: domestic, 148;
echoes, use of, 10; lu-
minescent, 19; nocturnal
vision of, 27, 32; orientation
in darkness, 27-28; see also
Bats

Antennae, 54

Apparatus. See Equipment

Aquaria, 26

Architectural acoustics, 50

Auditory nerve, 20

Barnes, H., 147

Bats, 2, 18, 26-32, 44, 121-22,
127, 148; acoustic orienta-
tion, 32; brown bats, 85,
88, 93; chirping bats, 87;
chirps, 87; ears, 28; echo-
location, 29, 78, 90-95, 99-
100, 112; fish-catching,
95-99; flight, 89-90; FM
bats, 87, 95, 97, 113, 116,
125; foods, 28, amount of,
88-89; horseshoe bats,
84-85, 87, 116; impair-

ments: of mouth, 29, of
sense organs, 28-29; jam-
ming, 103; nature of, 26-27,
31; navigation, 2, 27, 32,
108; objects, small, 104;
obstacles, 31, 93-95, 100-1,
discriminating ability, 103-
4; orientation, 32, 51-53;
orientation sounds, 29-31,
84-87; rhythm, 89-90;
sonar, 148; sound intensity,
97-98; sounds: frequencies,
2, 29-30, 84-87; vampire
bats, 85; whispering bats,
85, 87, 95

Beat frequency, 111-12

Beat note, 111-12

Bell Telephone Laboratories,
148

Biology, 10

Biophysics, 26-27, 56; defini-
tion, 132

Birds, 148

"Bird Sonar," 148

Blindness, 10, 17, 20, 129-44;
facial vision, 131, 135;
orientation, 130; sound pro-
ducers, 143-44. See also
Vision

Blindness, 150

Blind people, 18, 26, 148,
150; activities, 131; echoes,
24, use of, 67; echolocation,
73, 110, 112, 143; orienta-
tion, 44; orientation prob-
lems, 100; orientation
sounds, 107; sound dis-
crimination, 104-5; sound
fields, 51

Bowen, E. G., 147

Boys, C. v., 147

Brains, 10, 20, 56, 64

Brown bats, 85, 88; obstacles,
93. See also Bats

151

INDEX

Buoys, 58

Buddenbrock, W. von, 147

Burglar alarms, 51

California, 21

Cancellation, 44

Carthy, J. D., 148

Cathode-ray oscilloscope, 60

Chirping bats, 87

Chirps, 87

Clickers, 65-69, 72-73, 74-77;
and echoes, 66-69; fre-
quency ranges, 76

Communication, 19

Constant frequency pulses,
116

Constructive interference, 44,
48

Cornell University, 132

Cosmic rays, 35

Cotzin, Milton, 132, 133, 143

Creaking, 22, 23

Dallenbach, Karl M., 132,

133, 143
Darkness, 17, 27
David, Jr., E. E., 149
Dead spots, 50
Deafness, 148
Deep scattering layers, 110
Destructive interference, 44
Dijkgraaf, Sven, 31
Doppler effect, 113-16, 149
Duty cycle, 126-27

Ears, 36; bats, 28; human, 29,
45, 64, 107, 134, 147; inner
mechanism, 19. See also
Hearing

Earthquakes, 117; seismic
waves, 116-17, 119

Echo detecting equipment, 36,
107

Echoes, 10, 24, 44, 52, 56,
148; acoustics of, 57-73;
concentration of, 75-77; de-
emphasized, 64; definition,
44; devices, 65-69, 70-73,
74-77, 107; effects of, 47;
experiments, 65-69; func-
tion, 45; magnitude, 61, 63;
masked, 59; multiple, 113;
objects, 73, small, 78-80; in
prospecting, 116; reflections,
75-77; return, timing, 73;
scattered, 70; suppression,
64-65; types, 81; use of, 10,
17, 29, 58, 116

Echo experts: in air, 26-32;
in water, 18-26

Echolocating equipment, 36,
107

Echolocation, 21, 81, 149; in
air, 108; bats, 29, 78, 90-
95, 99-100, 112; blind
people, 148; definition, 18;
devices for, 58; discrimina-
tion problem, 119-20; fish-
catching, 95-99; moving
vehicle, 77-78; obstacles,
31; oil prospecting, 117;
outer space, 108; porpoises,
22-26; underwater, 108;
warnings, 73. See also Ob-
stacle detection

"Echo-Location for the Blind,"
150

Echo sounders, 58, 109; value
of, 110-11

Eggers, Friedrich, 54, 55, 56

Electromagnetic waves, use
of, 107-8

Electronic equipment, 10

Electronic hearing equipment,
30

Electronics, 149, 150

Electronic sonar, 149

Energy source, 149

152

INDEX

Equipment: communications,
3; detecting, 36; echolocat-
ing, 107; electronic, 10;
electronic hearing, 30; head-
phones, 3; hydrophones,
111; oscilloscope, 60; seis-
mograph, 117, 118; snoop-
erscope viewer, 3; under-
water hearing, 20-21; un-
derwater loudspeaker, 111.
See Instruments

Evolution, 104

Eyes, 36, 54, 147; see also
Vision

Facial vision, 131, 135

False bottoms, 110, 116, 117

Fathometers, 58, 109

Feelers, 54

Fish, 148; auditory sense or-
gans, 20; bladder, 25; de-
tection of, 58; echoes, use
of, 20; hearing, 19; noises
of, 20

Fishermen, 58

Fletcher, H., 148

Florida, 21

Fluctuations, 48

Flute, 49

FM bats, 87, 95, 97, 113, 116,
125

Footsteps, 107, 138

Frequencies, 20, 25, 29-30,
49, 72, 84-87; high, 72

Fundamentals of Sonar, 148

Hearing: aquatic animals, 19;
auditory nerve, 20; fish, 19,
20; humans, 19, 72, 99,
148, under water, 20, 111;
losses, 148; sensitivity, 46;
sound location, 140-42;
wave length, 130. See also
Ears

Hearing mechanism: in fish,
20; in humans, 19; in por-
poises, 20, 24

High-frequency note, 47

Horseshoe bats, 84-85, 87,
116

Horton, J. W., 148

How Old Is the Earth, 118,
119

Hurley, P. M., 149

Hydrophones, 111

Icebergs, 108, 109

Incidence, angle of, 77

India, 21

Insects, 18, 148

Instruments, 17. See Equip-
ment

Interactions, 35, 42-44, 48, 51

Interception, 18

Interference, 50, 149; con-
structive, 44, 48; destruc-
tive, 44

Jamming, 103, 148
Jurine, Charles, 28

Galambos, Robert, 29
Ganges River, 21
Griffin, Donald R., 148

Hairs, 53, 54
Harmonics, 49
Harvard University, 29

Lawrence, Barbara, 22, 23,
25, 99

Light, 18, 36, 42, 129, 130;
velocity in water, 19; in
water, 18-19

Light waves, 53, 80; reflec-
tions, 70

Liquids, 40

153

INDEX

Listening in the Dark, 10, 25,
99, 104, 148

Man's World of Sound, 149

Marine animals, 110

Marine aquaria, 21

Marine mammals: sounds, 21.
See Porpoises and Whales

Massachusetts Institute of
Technology, Radiation Lab-
oratory, 149

Maxim, Hiram, 108, 109

Mental telepathy, 137

Metronome, 75

M.I.T. See Massachusetts In-
stitute of Technology

"Modern Sonar Systems,"
149

Moehres, Franz P., 84

Molecules, 9, 19, 39, 40, 50,
119

Moon, 120; echoes, 147

"More about bat 'radar,' " 148

Multiple echoes, 113

Music, 49, 50

Musical instruments, 49

Natural selection, 104
Natural sonar, 148
Navigation, 2, 58; of animals,

148; bats, 2, 27, 32, 108;

radar, 147
Neutrinos, 35
Nuclear explosions, 117

Obstacle detection, 31, 132-
40; ears, 134-35; echoes,
138-40; skin, 133-34;
sound, 136-40. See also
Echolocation

Obstacles: for bats, 93; wires,
100-1

Oceanography, 147

Oceanography and Marine

Biology, 147
Oceans, 109; deep scattering

layers, 110; depth, 19, 40,

147; false bottoms, 110,

116, 117
Orientation, 10, 19, 27, 148
Orientation sounds, 84-87,

148; air to water, 97; bats,

2, 29-31, 84-87, 89-90,

97-98; frequencies, 84-87;

obstacles, 93-95; porpoises,

22-26
Outer space, 9, 120, 147
Oscilloscope, 60; screens, 120,

122
Owls, 27

Parabolic horn, 70-73

Pavia, 27

Physical Science Study Com-
mittee, 7, 8, 10

Physics, 10

Piano, 49

Pierce, G. W., 2, 29

Pierce, J. R., 149

Pinging, 112

Plants, 9

Porpoises, 18, 20-26, 44;
brains, 20; creaking, 22, 23;
echolocation, 22-26, 99;
hearing apparatus, 24; lan-
guage, 21; objects, small,
104; Schevill experiment,
22-24; sonar, 148; sounds,
21, 22, frequency range of,
25-26

Probing signals, 18, 22, 23,
112

Psychology, 10

Radar, 18, 107-8, 120-27,
149; uses of, 120; wave
length, 121

154

INDEX

Radar System Engineering,

149
Radioactivity, 149
Radio waves, 18, 35-36, 122
Rayleigh scattering, 125
Reflection, 75-77; angle of,

77; specular, 69, 77
Reinforcement, 44
Reverberations, 44, 45, 47, 55,

57, 113. See Echoes
Ridenour, L. N., 149
Ripple tank, 49, 52, 69, 73
Rummell, J. A., 149

Scallops, 147
Scattering, Rayleigh, 125
Schevill, Barbara Lawrence,

22; William, 22, 23, 25, 99
Sciences, 26-27, 56, 132; inte-
gration, 10
Science Study Series, 54
Scientific American Magazine,

148
Scientific history, 32
Scientific theory, 32-33
Seismic waves, 116-17, 149
Seismograph, 117, 118
Sense extenders, 51
Sense of obstacles, 131-40
Sense organs, 36, 54, 55, 56,

133, 147. See also Antennae,

Ears, Eyes, Hairs, Skin
The Senses, 147
Shadows, 42
Sight: wave lengths, 130. See

also Eyes and Vision
Sine waves, 36, 38
Skin, 133; pressure theory,

133-34
Soap Bubbles and the Forces

Which Mould Them, 54,

147
Solids, 40, 116
Sonar systems, 18, 107-8,

112-20; bats, 148; birds.

148; discrimination prob-
lem, 109-10; Doppler effect,
113-16; echolocation: ice-
bergs, 108-9, ocean bot-
toms, 110, submarines. 111;
electronic, 149; frequencies,
111-14, constant, 113-14,
rapid, 112-13; marine life,
110-11; natural, 148
Sound: airborne, 19; corners,
42-43, 130; discrimination
problem, 104-5; existence
of, 40; frequencies, 38, 41,
in bats, 29-31, 84-87, high,

29, low, 108, in porpoises,
25-26; indoors, 46-48; in-
tensity, 45; marine animals,
20, 147; multiple reflections,
44; nature of, 9, 41; orienta-
tion, 30, 148; outdoors, 46-
47, 48; pressures, 44; propa-
gation, 149; reflections, 44,
75-77; scattered, 70; trans-
mission in fish, 20; under-
water, 20, 149; use of, 19,
118, 147; velocity, 44-45,
in air, 65, measurements,
73-75, in sea water, 111, in
water, 19, 24-25; wave
lengths, 41-42, in water, 24,
25

Sound boundaries, 19-20, 40,

97
Sound-detecting equipment, 18,

19, 107-8, 112-20
Sound detection, 140-43
Sound energy, 19-20, 30
Sound-generating devices, 150
Sound theory, 134
Space, 40
Spallanzani, Lazzaro, 27-29,

30, 32, 87, 88, 108
Spallanzani's bat problem, 29-

30, 109
Specular reflection, 80
Speech, 49, 50, 148

155

INDEX

Speech and Hearing in Com-
munication, 148
Standing waves, 49, 50, 51, 52;

patterns, 51
Stop watch, 74
Submarines, 17, 109, 111, 112,

113
Supa, Michael, 132, 133, 143
Suppressor mechanism, 64-65
Surface tension, 53, 147
Surface waves, 53; use of, 54

Tape recorder, 46, 47, 48, 50;
reversed playback technique,

62-64, 66
A Textbook of Radar, 147
Time, 36; measurement of,

149
Titanic, 108

Underwater hearing equip-
ment, 20-21, 22

Underwater loudspeaker, 111

Underwater sounds, 20, 149;
discrimination, 110; energy
of, 25; frequencies, 112

Underwater sound devices,
109, 110, 111

Vacuum, 40
Vampire bats, 85
Venus, 120
Violin, 49

Vision, 22; in water, 19. See
also Blindness, Eyes, Sight
Voltmeter, 47

Walter Reed Army Institute of

Research, 29
Washington, L., 150
Water, 18, 97, 116; clear, 19;
dark, 18; sound in, 25; tur-
bid, 18
Water beetles, 80
Water waves, 53, 69, 80
Wave lengths, 25, 26, 29, 41-
42; frequency, 72; under-
water, 112
Wave motions, 9; property of,

36, 69
Waves: electromagnetic, 107-
8;light, 53,70, 80; radio, 18,
35-36, 122; scattering of,
80; seismic, 149; sine, 36,
38; sound, 18, 53, 149; sur-
face, 53; use of, 17; water,
53, 69, 80
Whales, 18, 148; brains, 20;
hearing, 19; noises of, 20
Whirligig water beetle, 53-56
Whispering bats, 85, 87, 95
Witcher, C. M., 150
Woods Hole, Mass., 22
Woods Hole Oceanographic

Institution, 22
World War 11, 111, 120, 121,
149

X-rays, 119

Zahl, P. A., 150
Zoology, 10

156

MARINE

BIOLOGICAL

UBORATORY

LIBRARY

WOODS HOLE. MASS.
W. H. 0. L

ECHOES OF BATS AND MEN

In 1938, Donald R. Griffin, then a senior at Harvard, took a cage
of bats to the physics building, where one of the first laboratories
for detecting ultrasonic sounds had been set up. There, for the
first time, the high-pitched clicks by which bats navigate were
heard. This pioneer experiment has touched off ever-widening re-
search in physical biology, research which has included the investi-
gation of the navigating techniques of animals. Echoes oi Bats and
Men is a brilliant report on how studies of bats, porpoises, and
whirligig beetles and of electronic radar and sonar are now expand-
ing man's understanding of physics. Dr. Griffin shows how this
knowledge may be applied to help the blind "see."

^^^^^^^^^^^^^^V j.JiM^'mL

^^^T^cienc^fuoj^Series is part

^^^^^^^^^^BlW »nil!jLJo!

of a dramatic new program for

the teaching and study of

physics, originated recently by

[^■p^^mH^^B "^is^ ^M^B^^Pll

distinguished American scientists

1 W*ii^^r^^^!^^^^_^.:^^^^^^^^B

and educators meeting at the

Massachusetts Institute of

I^^H^^ jV'^HI

Technology. This Series of

up-to date, authoritative, and

1 ^^Rh!Bv. ^^h ^^^^KtiJkS\

readable science books is

^^^^B ^^^^^^^m jt ^^^> i^S^^^S^^K ^^^^^^^^^^^^^^^1

prepared under the direction of

■■■■ «■■■■* »^j0» .Jitiit >« 4«HBwVlv ■■■■■■■■■■RH

the Fhysicd Science Study

Donald R. Griffin was born

Committee of Educational

in 191 5 in Southampton, N. Y.

Services Incorporated and is

He grew up mainly around

published in co-operation with

Cape Cod and was educated

Doubleday Anchor Books and

at Phillips Academy, Andover,

Wesleyan Universit) .

Mass., and at Harvard

University. Dr. Griffin taught

physiology and zoology at

Cornell University until 1953,

and since then he has been

Professor of Zoology at

f r\ n 1

Harvard. His widely acclaimed

book Listeiung in the Dnrk

1 1

was published

V^f^y

in 1958.

A DOUBLEDAY ANCHOR ORIGINAL

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